@rek that’s basically what I was doing with the valve stem. Having a premade one-piece cap like that one would have saved all the headaches. I was using a regulator I found at the hardware store designed for using paintball tanks with air tools. It was a really compact setup.
@jkaniarz forgot i had a photo, my 'keg coupler' is a bit cheaper-looking than the one I linked (was in japan at the time, and couldn't find all of the exact same parts hehe)
So yea, I'm re-using the same regulator with the paintball tank, so the adapter is to couple those together.