Coleman 5-Gallon Solar Shower
https://www.amazon.com/Coleman-2000014865-5-Gallon-Solar-Shower/dp/B0009PUT20
A five gallon dark plastic bag you hang in a tree and let the sun warm up the water.
Got one and the outside shower house. Found it easier to just use rain water outside from black trashcan. In winter my canning pot at the dry sink works good too.