Not Easy but enlightening
Behind my Roadtrek wardrobe was bare fiberglass wall, no window.
I began building a wall using ABS plastic from a plastic supplier. Cutting it down was no picnic but the hand metal shears were useful for the finer parts.
Sprayed foam backer and plastic with Weldwood Top and Trim.
Threw plastic down on foam. Good stick. Trimmed around it.
Next did the same. Threw the whole thing down on the vinyl I chose. Trimmed around it.
I glued wood blocks to the wall in strategic places (also held by acoustic tarry mat). Added insulation-fiberglass was easier to work around blocks..
What I didn't plan for was the inward curve the plastic followed in the van thus wrinkling the vinyl job. Just like the back of your hand as you bow back your fingers. Sooo... you can separate the pieces some with a ruler or by hand and tighten and do over. I used vinyl covered buttons with screws off Amazon.
Another difficulty is getting your wood blocks lined up where you know where they are when you go to screw the whole thing down to the wall.
Conclusion: Probably worth it as upholsterers are booked through the year. An interesting process, not beautiful but not bad at all. I've yet to drive with the new wall. We'll see.