Cardboard...sweet. Well, I'll start the replies with my thoughts and opinions. I would use steel. Why? For me, it's more readily available and cheaper, I've been getting the hang of forming it (I go to school for metalsmithing and metalworking, so I get graded on this) with hammer and stakes (hand dollies), and I just like it. Both steel and aluminum can be strong and durable, but aluminum, you hanve to have thicker gauge material. Heat affects steel and aluminum in different ways. It can harden or soften steel, depending on how you treat it after the heating. I have taken a piece of 16ga mild steel from the shelf at a metal supplier, and gone from a flat 9" diameter disc to a goblet shape that was 5" tall with a mouth 4" across. I wailed on this thing, never normalized it (aka annealing or softening) it, and never had a crack or overlap happen. I worked both in a stump and over stakes (metal forming tools), but man if you could get some thicker gauge Aluminum to do this with, you might be better off. Softer, more ductile, and you'll probably just thank yourself later. Weld it or rivet it. Sheet metal screws would even do the trick. A really easy thing to do is find yourself an extra seat pan for your bike, and use that as a base. All you would have to do then as shape the rear hump and you'd be set. None of that fussing with locating/drilling holes for hinge plates, rubber bumbers etc... As for the foam, try to find the "sandable" product. I don't think "great stuff" sands well (full of large bubbles IIRC) Blue or pink insulation foam works really well. Just spray adhesive or rubber cement it together and it is rock solid to work with. Goog luck and keep us posted.
-Chris
