A cautionary tale of using aluminium inner panels. A while ago while I was involved in novice rally a guy in our group made lightweight door cards from alu plate. Yes it did look cool but after a spill on a rally leg, he slid his car sideways into a tree and pinballed off it. The door card popped free of the door and sheared off his elbow. It was nasty. He would've taken a beating regardless but the door card did the majority of the damage to him. Fiberglass would make for a cool looking moulded panel and you could spray it with adhesive and wrap it in felt etc plus add provisions for a valet light and door speakers. Seen some interesting spins on custom door trims using fiberglass but the stuff is kinda heavy. What about MDF and wrapping it or maybe a sheet of ABS plastic? ABS can take a hit, is waterproof and pretty easy to cut, shape and drill. Stuff is pretty light too...