I am also interested in doing this. My car is going to need some serious tlc before I can even think about putting up on a rotisserie. I have some ideas for making something modular. Basically I want to start with just a basic platform built up from 2x4x0.188 rectangle tube. From that I was thinking about making adjustable extensions for the front and back. They'd be removable and the whole thing would break down into something that could be leaned up against a wall or otherwise "put away" when not in use.
Would people be interested in a collaborative design?