I've was thinking of building my own since my Coleman On Demand unit has proven itself fallable at the most inopportune time...
At first I thought the battery was bad, but the damned burner goes out instantly right after ignition... Might be a stuck impeller on the pump. (After scouring the internet and reading scores of complaints about the same problem I have)
Will just have to tear the damned thing appart to know for sure.
Besides a burner about all you'd need is a pump and a coil, but then I saw a simple little setup that a father and son from New York had at IRG2.0.
It was a garden sprayer with a shower head and hose.
The dad had a mark on the side of the jug, which showed how much hot water to add, which he'd heat up on his camp stove... After that, he'd just top it off with cold water to get his desired water temperature.
A moment or two of pumping and they had a portable shower... No batteries or anything else to worry about.
A pretty simple, fail safe system if you ask me.