Aaaahhhh I'm a bug don't spray meeee ahhhhh!
Actually there have been a small number of threads in similarity to yours. Some were painted black for heat absorption. Better if you had someone switching off to pump while you shower. I use an Eccotemp on demand hot water shower. They're simple and cheap. I paid $139 for mine two months ago. All you need is a water source with a minimum of 35 psi which you can achieve with a sprayer tank hooked up to said shower. The tank and pump I picked up at northern tool for $169. Power is from a battery and either solar or shore power. I realize you are going for the inexpensive and space saving route, but I thought I'd show a little of what I did in case you ever might want to upgrade. I also am a landscaper and have used similar products for which you are thinking of trying, and personally IMHO I believe it would be inadequate for a really good shower. I do wish you luck though!