Utterly Mad

or cardboard. I'd say aluminum frame, but aluminum is a bitch to weld. Cast iron is a no go because of weight and the brittleness of the material. You can't really bend it, a good sledge hammer blow will usually break cast iron. If you want that bitch to move, I bet you could rig it up to a bicycle, modify the bike with training wheels or something and pedal slow.

Seriously though, a double or triple layer of cardboard is about as light as you are going to get and will certainly block those little plastic pellets.