Boy, did I overthink what I wanted to use for the walls!
Materials I considered using for the walls: stretched canvas, masonite, 1/4-inch plywood, shower wall, and finally, door skin.
Door skin is essentially a very thin plywood, about an 1/8-inch thick. It had a better, lighter, and smoother feel than masonite. It was somewhat flexible. And it was easy to cut. Not as easy as some of my art school friends suggested it might be – it was thought that I could cut it with a sharp utility knife, but that was not the case. I could cut it with a small hand saw very easily.
Because the walls bowed in both the vertical and horizontal directions, the hardest part was measuring.
The back edges were also sloped at an angle from top to bottom, and I rounded the corner at the top in the back top.
In the photographs below you can see that I ended up with full length 8-foot sheets that were trimmed to fit the height of the space, as the upper walls, sitting on the wood stud. I tapped brads into the top of the stud to hold in the wall in place on the bottom (the bed platform would be covering them eventually) and I screwed the top of the wall onto the top stud bar, being very careful not to drill in too tight, because it would crack the wall around the screw.
It took about 4-sheets of door skin to complete the walls. The van was longer than a sheet of door skin, so I joined the the sheets with a PVC joining strip.
Cutting around the wheel wells and around the gas tank was the most difficult to get right. Even though both of these spots were going to be hidden from sight by the bed and the counter area, I still wanted them to look good.