I like the shipping container suggestion, though those might be expensive to come by deep inland in Mexico.

Whatever the materials, the "shelves" do not need to be "shelves." Just some sacrificial liner on top of the metal crossbars, such as short lengths of 2x2/2x4 wood, or UHMW plastic (best!).

Plywood, or corrugated steel, would work for the outsides, if painted regularly.