If you goto a place that sells plexiglass, they typically supply aluminum skinned plywood, thin aluminum on both sides, and i think you can order it one side only. They use them for making commercial signage, 1/2" thick plus aluminum thickness. Then if you want the diamond plate look, the same store should be able to order you rolls of diamond plate pattern vinyl you can roll over it for the look of diamond plate aluminum but at a fraction of the cost.
I bought this aluminum skinned plywood to build the bathroom stall inside my race trailer and will be buying a few more sheets to do built in cabinets in there as well...so if you've ever been inside a enclosed car trailer with cabinets...it's the same material those cabinets are made out of.
Menards also sells a plastic diamond plate material that you glue to the wall like FRP board, looks nice and i almost bought it to line the lower part of my trailer walls but opted for carpet instead for door padding.