Sturdy plastic bag camping showers abound of course, but these hold only 20 litres, and can be heavy to lift up into position. Further more they may become dangerously hot, and sunny positions are not always the most private for later ablutions either! Having a hose fitting allows a length of garden hose to be run into a discrete location (perhaps even to an indoor bathroom ?)
More ambitious plumbed in solar water heating installations can easily cost $$$$,& require pumps or well placed header tanks. Such systems annoyingly work best at just the sunny time of year when you may be on holiday elsewhere-or when you are short of gardening water that could be doing double duty solar showering! This simple one can just be rolled out as needed.
If threaded stock is not available,an old tap can provide both a useful threaded 15mm pipe and the hex screw rings. Hacksaw free this off cut, then drill and file the plastic shell base to suit. Remove any metallic burrs & sharp edges.
Ensure any plastic top cover is well secured against wind and animal damage! Although screwing down may tempt, this makes cleaning harder,so consider a lift off wooden framed support.
A bottom layer of gravel-or even black paint-may assist with solar pickup (and stability in winds),but water itself is a strong infra red absorber.
Under clear skies,with max air temp. ~25 Celsius,no wind & VERY bright sun,60 litres of tap water (initially at 20 Celsius) warmed from 11 am at ~4 degrees C an hour, reaching a very cozy 42 C by 4pm. By mid night,under a ~15 degree evening, this had cooled back to 22 Celsius.
Using Q = m x c x deltaT, then 60 x 4185 x 22 = 5.52 Mega Joules of energy gained. At ~US 20c a 3.6MJ "unit" this is worth just ~US 30 cents,(& over 100 days of summer capable of saving US$30 = ~paying for itself), but the "solar shower" convenience may be much more valuable, especially off grid.
The benefit of dirty feet and salty body washing remaining outdoors of course are near priceless for harassed summer housekeepers!
This one has been in the kids tree hut since 2000, and is covered with perspex for safety. rather than glass. It still works a treat - on Jan 27th 2013, with afternoon air temps ~ 25 Celsius, the ~100 litresof water within had warmed to an almost too hot 48 Celsius by 5pm as the sun left it. ( The location was Wellington, New Zealand lattitude 41 South - Dec-Feb is NZ high summer of course)
Conclusion: The approach offers a versatile, educational & cost effective approach to DIY seasonal solar water heating, with temperatures to blood heat (37 Celsius/98 Fahrenheit) readily achieved- even under non optimum solar conditions. The "backyard benefits" of solar heating significant volumes of water to modest temperatures outwaeigh heating a small volume to a higher temperature. Water rarely will get to an unsafe temperature, making the setup safer for children too.
EXTENSION: Such enhancements as side insulation, double glazing top cover,heat exchanges, water depth & perhaps even "solar salt ponds" (or solar PV powered pumping) could readily be explored for greater performance.