While you're ripping hoods and trunk lids off old junkers, you could probably make something serviceable out of the doors as well. The roof too, if you've got the tools to tear it off.
Factory scrap could work too. My workplace uses enough metal bobbins, pallets, and other assorted materials to bury a house, and as durable as that stuff is, eventually it gets too beat out of shape to use in production, at which point you simply have to toss it. Since it's otherwise destined for a landfill, you could well convince them to hand some of it off to you for cheap. I guarantee you a metal pallet can take more of a beating than a couple of car hoods ever could.