Here's a few ideas for you.
Many Vanners have made up molded fiberglass panels for the rear corners. This leads itself to molded-in rear speakers.
The original factory rear corners in the deluxe vans were made of a thin fiberboard, like Masonite, which appeared to have been steamed into the curved shape, then was covered with vinyl. The top corners were sometimes made of molded fiber reinforced plastic:
I'd suggest making mockups of cardboard; you can curve, trim, and adjust as necessary.
Try looking through the images of van interiors on the sister site,
www.showvans.com for more ideas. Some are pretty brilliant.
Many conversion vans used wooden or molded ABS plastic inserts to take up the space between the walls and the windows. These are some I pulled out of a high-end van:
I don't recall whose picture this was, but you can see how linear braces can be used to get away from curved walls, while providing you with lots more points for secure attachment:
In my old Dodge van I used 1/4" tempered Masonite for its curved walls. It was about as rigid as I could bend without the sort of notching Reed described.
Hope that helps a little!