This is a recurring question in camping forums. My recurring answer, so far, is that there's no need to reinvent the wheel, or the camp shower, both have already been done ---
The simplest, easiest, most foolproof is the Zodi Extreme Shower hand-pump-pressurized canister with its hose and combination on/off valve/shower spray head. Heat some water either directly in the canister on a camp stove burner, or simply add some boiling-hot water (boiled in whatever container you've got, from whatever heat source you've got) to some cool water already in the canister, adjusting the proportions of hot and cold to taste to achieve the "Goldilocks" temperature... You don't need any of the other expensive, fancy, Zodi apparatus. I have a simple solar shower sack, which works great in ideal conditions, such as when I used it last Sunday afternoon, on a hot, bright, day. But when you need to add heat from combustion, the Zodi pump-spray shower is unrivaled in simplicity, efficacy, and reliability. Occam was right --- the simplest solution is always the best bet!