I just have to say here..Killer Klowns From Outer Space. Awesome movie lol. But seriously I think I have some things that might help. Google plans for a home made teardrop trailer. Using that as a starting place you can modify the basic plans to make a clown car. I think something like a Volkswagon Beetle would be cool, but a bunch of creepy clowns in a HEARSE. That would be something! Maybe try a platform of plywood on free axles to start, then a frame of bent strips of ply or very small diameter plastic pipe. Then skin it with more ply or something else thin and bendable.