ive been working on my idea of this for some time now. things ive learned that will give you a major head start on building your own
4 inch abs black plastic pipe is 18 inches to a gallon.
shraeder to 1/8 npt adaptor is great for pressurizing.
i also put a vacuum break/fill port at the top. when you just want a trickle of flow.
when choosing valves for this look inside the valve for the restriction. meaning not all valves are anywhere near full flow some have holes small as 1/4 in yes that could explain why the garden hose in the back yard sucks compared to the front yard. boiler drain valve was the biggest orifice i could find.
its a good excuse to buy that new drill bit and 1/2 in tap for the valve or 3/4 in or you want
i found a 9 position flower watering wand that has a fireman style valve. big lever set and forget no need to hold it to spray some rope to an awning pole or tree and great shower.
thread cap covers are a cheap investment to prevent gunk/mud and for the vacuum break/fill at the top
2 4 inch caps lapped and glued flat to flat make an excellent guard around the handle and valve. do a lil notch for the spigot and never worry about another handle breaking off on a branch again also gives more interface for the tapped npt hole for the valve
i also went over board have an rv pump and a 10 plate heat exchanger can take cold glacier fed river water into a warm shower on lower volume at high volume its luke warm. if pressure and flow is important get the 20 plate system. on amazon under dudadiesel