No personal experience here, but I'll list some thoughts.
Maybe use 4x8 sheets of plastic paneling? I know Menards and Lowes sell it near their bath stuff. It's probably about 1/8" think and fairly flimsy, but if you use green treat ply for backing, or maybe even the fiberglass impregnated wallboard that has the gray plastic layer on top (used for tiled showers), you should be fairly solid I'd expect. Good caulk on the seams, and maybe even a painted on rubber underneath the plastic panels?
The steel roofing stuff would look cool I'd expect. Though probably more expensive.