I spent a significant portion of my recent life living in a van, nomadic and driving as far and as often as fuel prices and weather permitted throughout the American Northeast. It was an interesting (and cold, wintery) time, and one born of circumstance, calamity, and character-testing chance. Amongst other things, it challenged my devotional practices, ritual observances and prayer cycles, which for years leading up to that were satisfied in a full-time dedicated Temple. In these months, I learned mobile and slimmed-down tactics, adapted from previously developed short-term tricks for traveling and engaging devotionally in the wilds. Not all religious practices are easily made to move about in small-form. Yesterday, on a social media site, a co-religionist of mine posted an open question to the community, asking about suggestions for and experiences with altar boxes of a portable, easily stowed variety. Below is my (hopefully somewhat helpful) reply, which I share here in case it is of use to others.
First, I would encourage a differentiation in language, between altars, shrines, and so forth. Certainly they can be integrated, as is often the case, at which point I think it appropriate to use language to reflect this. (For more on this, read my article on their differences, as well as an exploration of shrine rooms, Temples, and more.) In short, an altar is the place where you *do* stuff, and a shrine is a place where you *honor* stuff.
That said, in either case, the logistics of a small and portable option can be as simple as defining the space limitations present, and intended or desired types of use. For example, for the ultra-portable, secret, and literally pocketable, an Altoids tin can work in a pinch: tightly folded cloth, tiny little set of bowls/cups (think doll-house size!), and small vials of libations, oils, or incense. The rimmed lid makes it easy to put icons/images in, and a small "stand" can be made out of something as basic as shaped popsicle sticks, to keep it upright when opened, or a bit of silk ribbon diagonally from the inside lip of the lid to the inside lip of the bottom, like an oldschool suitcase, to keep it from falling all the way back and allowing gravity to hold it just so.
Scale up from there to a cigar box or the like, and the same basics apply. Cigar boxes come in a variety of sizes and styles, from plain rough wood to fancy foil-trimmed extravagance, and have the added benefit of smelling wonderfully of Spanish cedar and tobacco.
If what you're looking for is a portable worship space, I recommend building something akin to a portable Catholic mass kit (you can find ample pictures online), which can be fashioned by the crafty sort to fit in cigar boxes or shaving bags. (Remember: crafting can, and often "should", be a devotional act! Sanctify your crafting time with prayers, offerings, and acknowledged engagement with the gods or spirits who govern such undertakings in your household or personal cultus.)
Alternatively, if discretion and/or secrecy is required for added privacy and security but size is not as much of a priority, purchase second-hand an oversized dictionary or similar, carve out the interior with a rotary tool, and glue the pages (from the inside) with a heavy adhesive. Take a piece of closed-cell foam from the craft store, carve out shapes needed to hold all of your small travel items, and then glue a piece of satin to its face. Have a soft piece of open-cell foam (the soft cushy kind) of the same dimension glued to the "lid" of the book-box, which will press down the contents and keep them from jostling/breaking in transit, or when stored upright. This design allows you to use the surface of the book itself as your "work space", and with the right things placed in side -- such as a cloth, candle, and an icon -- can be quite beautiful. As it sits on a shelf, or in a book-bag, or at a bedside table, the book blends in as just another reference tome, but in half a moment can be converted into a moderately sized, space efficient space for prayer, ritual, meditation, and more.
(Books also carry a bit of a nice touch, assuming the deities in question aren't offended by the destruction of the printed word. To some gods and in some traditions, defacing a work of printed words and knowledge would be a *grave* affront; it is wise to refer to a diviner or somebody with experience dealing with the deities in question, to make sure that your crafted portable ritual space does not carry such offense. When no such offense is risked, a book thematically appropriate to the gods being honored could be used - such as a book of love poetry for a god of such things, or a book of military history for a god of war, and so forth - and the contents of the book which are "carved out" can be given to Them in fire, as a material offering.)
Laptop bags and cases are often good for this as well and come in both hard and soft shell structures. They're fairly rigid in construction, can be made moreso with dimensionally appropriate hardboard or pegboard, which is cheap and only a quarter-inch thick. The same trick with the foam (above) can be used here to create perfect carrying compartments/inlets for fragile or particularly sacred items, while the versatility of it being literally a bag makes it both *not* stand out as obviously religious in nature, and also incredibly easy to carry (by handle or shoulder strap), as well as tuck into a larger bag (such as a backpack).
If you're looking for something larger, less portable, but impermanent and easily stowed, take a look at mobile-home or tiny-home furnishing modifications. Tables/surfaces that hinge up against the wall (murphy-bed style) are fantastic. When I was living in my van, I designed -- but did not wind up building, due to getting into this house -- a shrine space that would be housed entirely in an old-school pub dartboard "shell". The interior dartboard itself is removed, obviously, and that leaves a wooden wall-hanging box with "church-like" double-doors, and space designed to "hold stuff", on both the insides of the doors and the interior foot-print. My plan called for a bottom-hinged drop-down surface (like the one pictured above). I would have various cloths to put on this for different cycles of ritual/holy days. Side compartment with foam, etc, for holding scaled-down/small statuary or icons, and small vessels for offerings, candles, etc. I was going to mount it in the window of my van, which was recessed enough to take the unit easily. I will probably still do this, as my van hasn't gone anywhere, but the last months have seen me preoccupied with rebuilding my full-scale shrine complexes in my new home.
(Alternatively, if such a structure cannot be "mounted" or "hung", it could be supported on a portable easel/tripod, which would fold up or collapse for easy storage.)
On that same level, tackle-boxes, tool-chests, and crafting or sewing kits are all VERY good and easily adaptable, if small/ultra-portable is not the direction you're going. Things like this wooden sewing box are wonderful, sturdy, attractive, affordable options that come in a variety of styles and colors. They have multiple staggered levels for both storage and "pop-out" or recessed shrine/altar space, or can be purely storage on the inside have their top be turned into a work-space once you unpack your kit form inside.
Here's an example of something historic and Catholic that you could do a similar style of using the dart-board-box above. If it is a matter of space, again, this could be set with tripod legs, or even just a fold-out triangle-strut back (like a tabletop picture frame) to go on the surface of any table, desk, or night-stand -- or just on the floor. Any pillow can be used for prayer kneeling or meditation in front of or below it.
Also, never underestimate the power of battery operated LED candles or brighter "spotlight" style lamps, which can really light up a worship space when flames are not permitted. Similarly, if you can't make burnt offerings with incense, plug-in oil diffusers also do well for dispensing fragrances. When open flame is not an option, electricity will often be "received" as a form of fire, when used right, although do check with your deities and spirits first to ensure that no offense is made.
Also, it may sound odd, but don't rule out rifle, pistol, or hard-shell guitar cases. They're all sturdy, spacious inside, and designed to protect important/valuable/dangerous items in transit. They can be stored under beds, in trunks of cars, upright against a wall, in a broom closet (ha) or behind a couch, etc. Lift up a Winchester shotgun case on a couple of 5-gallon buckets, though, and you've got a decently sized space for Work, or a really nice sized space for a "hotel shrine" (e.g. a devotional shrine with enough space for lots of images, statuary, and tools/accoutrements for an entire pantheon, which will easily break down again when morning/evening prayers are finished, or can be stably left out for a few days or even weeks at a time). The buckets are good for a base because they are universally shaped/scaled, cheaply available nation-wide, and great for taking any poured-libations, offerings, etc from the temporary interior religious space to an appropriate outdoor place, without needing to bring expensive bowls/vessels out and about. Walking around with a bucket is not as conspicuous as carrying around silver chalices or crystal bowls in most areas, and doesn't attract much attention. (You can "consecrate" the inside of the buckets with dedicated oils, etc, to make them suitable carrying vessels for the offerings as you transport them outdoors, or however is appropriate in your religious tradition or lineage.)
A large firearm case is heavy-duty (but light-weight), has open-cell egg-carton foam throughout, and is durable enough to transport an arsenal of ritual steel, and definitely suitable for icons, carved wood/bone items, and so forth. It even does well in transporting things like feathers, which would be easily crushed/damaged in other enclosures. Any "upright/standing" statue under about eight inches (unless it has a very large base, etc) would fit inside, I think, but best to measure across the widest point and compare to the case dimensions. Framed pictures or wooden icon images would have no problem in this, and paper can be easily pressed between thin rigid boards to keep them from crimping/creasing.
The possibilities are endless. I have seen powerful shrines made entirely inside of shoeboxes, which were left otherwise mundane on their exteriors, and stored secretly amidst other similar boxes in a closet, attracting zero attention. Hat-boxes, too, if a round space is preferred. (Hat boxes also tend to be made a little more sturdily than shoe-boxes, as they are actually intended for ongoing reuse.) Large or small, hidden or visibly extravagant, the methods by which you might craft tools and space to praise, worship, and invoke your gods virtually anywhere in your life, no matter what circumstances get tossed your way, are virtually limitless.
I am a Temple priest, and though I presently lack my Temple - funds and land need to be acquired for its rebuilding - time apart from the daily structures, rigors, and routines made possible by dedicated large-scale space for religious life and service helped me to find new, and at times humbling, ways of praising my gods. Whether a person is new to their gods, has been working with them for a lifetime, or indeed does not even know them yet by name, there are always options, always solutions, to finding ways to show them devotion, respect and praise.
