An outdoor shower enclosure gives you far more freedom of use for an outdoor shower. Instead of a place for a quick rinse, you can supply soap and towels, and use complete showering facilities. You can even install a simple solar water heater to provide you with a few minutes of hot water to bathe with. The following article makes the presumption that you have already installed the shower, drain, and floor.
Step 1 - A Basic Box Design
Your outdoor shower stall should be at least two feet wider than the largest person you expect to use it. A small stall could be as little as four feet square, but a 4 x 6 stall allows for more freedom of movement. The walls should be at least five feet above the floor, but may be as high as you feel is needed for proper privacy. Three walls and a shower curtain are all you need to have, but you can build the shower stall as complex as you want.
Step 2 - Prefab Shower Stalls
Instead of building your own, there are quite few prefabricated shower stalls available that can be mounted on a 2 x 4 frame. For most purposes, these stalls are more than you will require from an outdoor shower, but they will save you the time and trouble of designing and building your own.
Step 3 - Build the Stall
Build the frame of the stall by assembling the two sides, and then build the front between them, forming the frame of a three-sided box. Place studs in the walls at 24-inch centers, and install cross-braces between the studs halfway up the height of the wall. Cover the outside with T-111 or other outdoor siding, and use plastic or polyurethane panels on the inner walls. Paint or seal the outer walls, and caulk all joints, both inside and out to prevent damage from weather and insects.
Step 4 - The Door
Your shower stall door could be as impressive as a bat-wing, or as simple as a hinged sheet of plywood. For stability, use three hinges, with the top and bottom ones inset at least six inches. If you want something even easier, install a piece of PVC across the top of the opening and hang an ordinary shower curtain.
Step 5 - A Solar Water Heater
Paint sections of four-inch PVC a flat black. Wrap them with clear ¾-inch tubing so that the coils are 1/2 to one inch apart. Hang or mount the PVC in an area that gets full sunlight. Connect one end of the tubing to your cold water supply, and connect the other to a valve leading to the shower head.
For more hot water, make multiple coils, or install a tank and simple heat-driven recirculating device. If placed on a dark roof or other dark surface, the water will heat faster than it would on a light colored surface.