Back in my "homesteading" days in northern Idaho, we had an outdoor shower for about six months (no indoor plumbing at all - in fact, not really any indoors of any kind). Through the month of August we bathed in the nearby creek, but it started to get too cool for that as Fall came on, so I decided we needed a heated water shower. The shower stall was four posts set in the ground with a floor and three walls made of roughsawn white pine (I had a Foley-Belsaw sawmill powered by a 1952 International H tractor). It was open to the spring area where the chickadees could watch. The solar water heater was a 55 gallon drum painted black, on the roof in a box with glass on the south side. I eventually learned that the drum was too big and the limited surface area exposed to the weakening suns rays couldn't quite get shower-level warm by evening. By November, it wasn't even getting up to cool...

So, I scrounged up a 2-1/2 gallon electric water heater and plumbed it into the pipe from the barrel. That much hot water mixed with the appropriate amount of cold is just enough for a shower if you run about 30 seconds to get wet, then lather up at leisure with the flow turned off (25 degrees out), and rinse off in about 45 seconds before the water got cold. The big danger was slipping on the ice that formed on the shower floor...

You never change things by fighting the existing reality.
To change something, build a new model that makes the old model obsolete.
R. Buckminster Fuller