What about using a type of ABS plastic or something along those lines
ND's got a point. Awhile back on one of the so many car shows on TV the guys went to a p[lace that makes truck bed-liners and had a custom made ABS type liner. The forms are made of plywood and a sheet of plastic is put over it and vacuum formed. Just takes a matter of minutes. I would imagine that you could contact some of the bed-liner manufacturers, explain what you have and see if you could build the form, dismantle it, and ship it to them, or if demand seemed high enough they may be able to do a custom liner. It would be worth a shot at least. Also with an ABS type of liner you could possible make it a little more rigid and then have it upholstered. You could have any design formed right it when the vacuum was pulled. It is a thought.
HEY CBOY....check this out. Right in your own state. Might be worth shooting them an e-mail. No postage necessary!!