I was contemplating to have a temporary shower in my van but decided to just have the transom one. My idea was to have a round shower pan, perhaps custom with 1” or 2” lip. Design and built a circular, easy to erect, with corded poles tent wall to fit into the pan about 1’ high - Tent Poles
Mount a circular curtain guide on the ceiling with curtain going into the tent wall. Use marine shower pump to drain the shower pump.