Allen Wrench,
It is a wood frame with metal formed over it. My thought is, if I get at making one, is to cover the frame with 1/4" luan plywood and then glue a very thin metal sheet to the luan.
Something the thickness of stove pipe metal.
The frame is mortise and tenon construction with the stringers running the length of the body set into the upright ribs. This is very much like boat construction. The ribs are rounded over to relieve the stress on them.
Decide what body shape you want and cut the floor out to match then shape the bottom stringer to match. Since mine will most likely be a boat tailed design I'll make a top stringer to match leaving the tail ends to cross over to compensate for the shape of the body should the sides flare out wider than the bottom and I'll cut the doors in later. Shape the ribs to give the sides the shape you want.
Tops and bottoms of the ribs are mortised into the top and bottom stringers or you can notch them in like the stringers are. I have a mortising machine so it's easy for me. You can buy a mortising chisel for a drill press in different sizes.
I have a Grand Niece who will be three in May so I think a smaller edition of the one I want to build is in order first. She can tear up Grandma and Grandpa's back yard with it. If it were me and I hadn't done a lot of this type of work I would build a small scale model first to get a feel for it before I committed to spending a lot of money.
Steve.