Last year me and a guy called Willy built this rocket stove shower at the Koanga Institute, New Zealand. We got some 60 gallon oil drums for free from the local garage and used a bit of old chimney pipe in the shed. Old bricks, are normally lying around for a stove feed chamber (they were) and we had an L shape piece of welded box section from a previous rocket stove build that we used again.
The rocket stove L was inserted into one of the drums and back filled with pumice and sawdust for insulation. The feed chamber was made from bricks, and sticks out the front of the barrel connecting to the L.
We had an old water cylinder and inserted it in to the two drums on their side whilst propped up on a couple of spacers made from a metal bed frame and bricks. This created a 2 inch cavity around the water cylinder for the heat to flow around before exiting out the chimney.
We connected the cold mains water to the cylinder, turned it on and fired up the stove. Once the water in the cylinder heated up, the cold water running through the copper coil inside it was coming out hot for a shower.