I had a friend who needed a tub for medical reason, but only had a fiberglass shower stall.
Scrounging, I found some scrap aluminum rails. I cut and mitered them to fit in the stall, and screwed them into the stall with stainless sheet metal screws. I put lexan/polycarbonate over the frame, and fixed it with screws and nuts, also stainless. I sealed it with silicone sealer, and...it held!
From time to time, she would let the water run, but it held with about 2 feet of water in it.