I have seen MDF, regular old ply, cardboard and of course modeling clay used. What do you use? Got any build tips? How about pics?
My friend Richard Oakes has built many radical cars over the years and he method he uses is to make a series of profiles of the body shape every 12" or so in plywood (like slices through the body shape). He then nails thin wood strips (may be 1" wide by 1/4" thick) onto the profile. After that he smooths the shape with bucket loads of cheap bondo. At this point he can also put reveals and lines into it. He uses templates cut into thin plywood in order to ensure symetry. He then sands to smooth and sharpen. It is then painted and waxed ready for a mould to be made from it. He has done some very complex shapes in this way - at least one of which will be well known in the USA;
Small tubing, welding wire and duct tape. You can get some nice compound curves and intricate parts "roughed" out with these.