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.