You might try Home Depot of Lowes for a 4x8 sheet of poly board or other thin [1/8in or less], plastic sheeting. You could cut it to fit your dimensions, affix it to the walls, caulk the corner and bottoms. Kinda like your own custom made shower surround without much added weight. Not sure how expensive the stuff is though. I'd be leery too about water hitting the "walls". Good luck.
2012 Eagle 320 RLDS
2017 Ford F-250 FX4 Crew STX 6.2l