An On The Ground Tank System
Place a large drum or tank on the ground, paint it black to absorb heat from the sun's rays, and connect a pipe with a shower head out of the top and a cold water feed (which can even be from a garden hose) to the bottom. When the cold water supply is turned on, the water pressure pushes the warm water out through the shower head on top.
No moving parts or motors are needed. If the tank or drum does not have fittings on it to connect to a hose, cold water line or shower head, some welding will be required.
Up On The Roof
Mount a tank or drum on top of an outside shower stall or on an adjacent roof. Cold water is supplied to a connection at the bottom of the tank, and a pipe with a shower head is connected to the drum's top.
The drum is painted black. As with the above project, no moving parts are needed, as pressure from the home's cold water supply is sufficient to force water out through the shower head.
Water is very heavy, so be sure the drum is properly secured to the roof or to the shower stall structure.
Increasing Collector Efficiency
Mount a drum inside an insulated box to make your system much more efficient. The box can be made with plywood, and should be lined with an insulating material, such as Styrofoam.
Use caulk to seal out drafts and to keep heated air inside the box. In addition to the drum being painted black, line the interior walls with a reflective material to shine light around as much of the drum's surface as possible.
Construct a glass or Plexiglas cover for the top. Angle the glass opening so that it faces south, to gather the maximum amount of sunlight during the day.
The addition of an insulated box makes the finished unit even heavier, so physically securing it properly is important.
A Coiled Hose Collector
Coil a long garden hose in a spiral, using strips of wood or plumbers' metal straps to keep the hose together. This can be mounted on top of a shower stall structure, with one end connected to a cold water supply and a shower head or similar "sprinkler" at the other end.
The advantage to this system is that it's inexpensive, quick to make and not very heavy compared with drum/tank methods. However, not as much hot water can be stored--so while it is good for rinsing off, it isn't the best option for a long shower.