First I am retired sheet metal worker, so I had some thick copper sheet about 3/32" thick I cut the collar of the cap first formed it round to fit the needed diameter I then silver soldered the collar and a flat pc. together, next was to cut and grind off the edges making it round then came the brass lugs in the collar I used a couple of air line fittings leftovers, think they were 5/16" dia. Drilled and mounted in place then more silver solder, and then a male brass hose fitting for the hose draining or capping it for travel.
And last I glued a rubber gasket inside of cap, bingo let it flow!
It ain't pretty but it works.
Hope this helps.
I am thinking this is a part I could make and sell? Let me know if you are intersted!