Well, going back to boatbuilding technologies, I can see one of these 'Teardrop' trailers being pretty easy to assemble using plywood. No compound curves, everything is either a straight line or a simple bend. If you're handy with tools, just constructing the body shouldn't really take all so long. Chassis and wheels might take a bit of doing, though Northern Tools has trailer kits that I imagine would simplify things considerable.