You want a Carson Top!
Build A Carson Top
It's sort of the lazy hot-rodder's solution to chopping a top without reengineering the car, but the same sort of construction could help you here. Rather than having it upholstered, you could fiberglass it.
Nice MR2, by the way. I'd love to have one, but I really enjoy having a wagon (the 1 cubic foot trunk wouldn't do me much good).