Thought I'd start this Topic, as I have just answered a question for a guy on the "OTHER" site...here's my reply to his query about a shower pan....he was looking for an odd size (30 inch).
If this is not the right place to post this topic, let's move it to the right forum area.
consider what 'real' home builders do. Build your own shower stall/pan.
Use copper, galvy steel, EPDM /concrete, even have a plastic pan custom welded.........
Make a box from copper (have a sheet metal shop do it, with good soldered joints), and drop that onto a bed of ply. In the case of the bus, I'd recommend placing it on top of a sheet of rigid foam insulation, the pink stuff is rated for some loading, and is available 1/2 inch thick or so.
Have the 'walls' of the shower pan made about 6-8 inches tall, with the front edge 'rolled' or bent to a ledge to prevent cutting yourself on sharp edges. Leave the edges that will be covered alone, they will be hidden by the wall sheeting......the front edge where it's bent to form a lip can be supported with a piece of ply or Hardiboard (cementitious backer board) and the support material can be covered with tile, or something decorative and VERY waterproof.
Then, go to Home Depot or Lowes etc. and buy fibreglas sheet, comes in 4x8 size, and fabricate the walls of the shower from that. The pebble finish cleans up fairly easily come wash time. Comes in white and ivory colors.
Some suppliers also have plastic sheet that looks like conventional shower tile, look around, ask in your local home centers/builder's supplys. There's a LOT of stuff out there.
Note: I have NOT tried to curve this stuff around something like a Skoolie ceiling, tho I have read of someone using plastic laminate to do so. Maybe a heat gun will soften it enough, but that's an experiment I haven't tried yet. Next time I do an install, I'll try it and report back.
Use a fine-tooth blade in a jigsaw to cut the stuff, and take your time, GO SLOW. The embedded fibreglas is abrasive and hell on blades, so have quite a few spares.
I press the saw firmly on the sheet when cutting it to prevent the blade from 'snatching' it and breaking the blade or chipping the sheet. I also clamp a couple pieces of wood to the sheet to guide the saw and help prevent chipping or breaking the sheet.
I lay some blue masking tape down and draw the cutting line on that to make it easier, and it also protects the surface from the base of the saw.
Use the plastic corners and edge pieces they sell for joining the sheets, with a good grade of ELASTOMERIC-type caulk to make the joints tight. Take your time, a good job fitting/sealing it together will make repairs unnecessary later.
I DO NOT RECOMMEND USING SILICONE CAULK ON BARE METAL, AS THE ACETIC ACID IN IT WILL CORRODE THE COPPER. While it's fine for fibreglas and plastic, I can't recommend it for metal that is unpainted.
The 'glas sheets should cover the edge of the the copper pan, overlapping by three to five inches (3-5 inches), otherwise splash from the shower will get under it. I DO NOT caulk the sheets to the pan, as any water that gets behind the sheet will be allowed to drain into the pan instead of sitting there and corroding the metal.
I usually use a polyurethane adhesive when I install tub surrounds as a handyman. Holds like iron, tho sometimes I need to brace the sheets in place to ensure they stay put until the adhesive takes hold.
You can use copper pipe and sweat fittings to make the shower rod to support the shower curtain, looks cool. To keep the copper pipe new looking, I polish it and then apply a couple THIN coats of clear polyurethane. Just to the pipe, not to the pan.
Use 'roller' curtain hooks to hold the curtain without scratching the copper pipe.
You can also have a pan made up of galvanized steel, cheaper than copper, not as nice looking.......
An alternative is the same EPDM rubber roofing they use for houses and porches, just make up a stiff Plywood box, line it with the EPDM sheet (all joints glued properly, right?), and then pour/cast a concrete base inside the rubber "pan" (actually leakproof liner). Mounting the drain takes a bit of prior planning (it's cast in place when you set the concrete), but is common for homes. You can leave the floor concrete, or tile it, just like in the high-end houses.
Kinda tough to do right, but sure looks sexy when it's tiled.........
While a bit heavy, you can also use concrete board/sheet (Hardibacker, one brand name) to make the walls up, and tile them. Just ensure you have a VERY RIGID structure, as vehicular movement will ruin all the tile joints and grouting. ONLY use the correct fasteners to mount the concrete board as recommended by the manufacturer, as other screws (like galvy deck screws, etc.) WILL fail. I actually can't recommend tile walls, as they are prone to falling off from adhesive failure. Houses don't move or vibrate much, except maybe in California or Alaska
Articles on these types of shower pans are in "Fine Homebuilding" and "The Journal of Light Construction" magazines. JLC in particular has a CD-ROM of all of their articles you can buy for relatively cheap $, it's a tremendous resource for Skoolie builders as well as for those non-mobile things called houses.
Another source is "This Old House" on PBS, they also have a tremendous on-line resource collection of materials, tools, and suppliers. Google it.
If you still want a plastic shower pan, go to a plumbing supplier. They will have what you want, UNLESS the pan is an odd size.
In that case, have a pan made up.
If you look around, custom plastic fabricators can weld up a pan of poly for you, just like they can weld up custom tanks. Give them a try, just don't scream TOO loudly at the prices.
Make sure any pan you install has a small amount of pitch or slope to it to ensure proper drainage.