This would be a pain to assemble and take down every time. Maybe just hook up the shower rings ($1) permanently to the roof of the van and then thread in the shower curtain ($1) when showering.
But I was thinking about using just a wide plastic container and a Coleman camp shower ($15) and be careful not to spray water outside of the container.
Placing it behind the windshield on the dash should turn it pretty hot in a few hours.