the skateboard idea isnt a bad one.. i mean, not actually using a skateboard, but it does give me an idea of using thin pieces of wood to make a laminate...
i could layer thin pieces of wood on the frame, with a layer of glue in between each layer, then use a canvas or something like that and some bungee cords to press the layers together... when its hardened the lamination would be a lot like a plywood that would take on the shape of the frame...
then i could get a 2 or so inch thick piece of high density foam cit to shape, sew together a seat cover to stretch over the seat pan and glue down to the bottom of it... seal the wood before putting the seat together of course, and install a trim piece on the bottom to hide the edges of the seat cover
the laminated wood would probably be sturdier than a thin piece of sheet metal and gives a more custom/rustic aspect to the bike
as for the seat cover, someone could make a seat cover like sirdaft has in his photo... someone with the proper leather stitching/sewing equipment could make the ridges like that in the top of the seat.. probably wouldnt even have to use leather.. anything waterproof should be good too, like neoprene or vinyl and be lower maintainence, though wouldnt have the quality of a good piece of leather... maybe ill cure a piece of deer skin this fall for that purpose
never noticed before, but my CB550F doesnt have any loops like that on the back of the frame... im definitely going to have to change that.. i have access to an arc welder, no MIG
so.. taccuardi, there are some ideas for you for fabricating your own