I decided to go with 18 and 20 gauge steel sheet for the entire bottom side of the camper box. It’s a bit heavy unfortunately, but it’s pretty easy to deal with (no glue or fasteners, just weld it on) and relatively inexpensive ($28 for a 4x8’ sheet at the moment). For this back section the 4x8 sheet is pretty close to the right size so I will trim the edges after it’s welded on, rather than cutting it to size before hand.
