I forgot where I found this solar-shower-on-the-cheap. Probably Pinterest. Looks doable. It will be another project. It doesn’t look like it would last too long, but is as cheap as it looks.
PHOTO 1: Cut a 1/4 inch hole in the inner tube. PHOTO 2: Take a 1/2 inch pipe coupler and cut off it’s lower half. PHOTO 3: Spread a good amount of rubber-sealing compound on the small ridge of the coupler. Insert into the inner tube’s hole. PHOTO 4: Put some rubber-sealer on your finger and coat the outside area. PHOTO 6: The portable solar water heater can easily be emptied, rolled up, and tucked into a backpack or satchel.
Another suggestion is this: Just a quick tip on this for sunny days. If you will be away from your car during the day, put 1 or 2 containers of water up on the dash in front of your sun screen before you leave, and you will have hot water for bathing when you get back. If you paint the water containers black, it will work even better.