For insulation, ordinary fiberglass batts held in with screening/netting works as well as anything and is a lot easier to curve than sheet foam board used in the flat walls of most RVs.
My Coachmen (with a bunk in that area) originally used fiberboard/pressboard covered with headliner material and bent to shape. I replaced it with a flat piece of luan, stiffened with battens on the back, and finished with thin carpeting. This panel is attached to wooden cleats with screws (with heads colored to match the carpeting--they actually bend in very well). The carpeting I actually cut from an inexpensive indoor/outdoor "rug" sold by building supply big box stores (unlined, made from recycled plastic) and the material works very nicely for this application. It's kind of like the sort of carpeting one would use to cover car speaker enclosures and that sort of thing.
I suppose one could use spray foam for insulation, but that would make any future repairs to the area that much harder. The foam would stiffen and help support the fiberglass skin, unlike fiberglass.
