Old Mechanic -- If you can find a truck parts store, swing by and price a sheet of 'swing-door' material. It's used to made the back doors of box van trailers.
Plymetal is an example. Local to me, it's about $100 a sheet.
So then you have a mill-finish aluminum deck, just soak the edges with epoxy resin to seal them, and you should be good to go.