: Perhaps you could make a pressure chamber from some 3 inch or 4 inch sched.
: 40 PVC plumbing pipe. . . . A brass filler stem from an old bike inner tube
: is glued into a fight fitting hole drilled in the cap.

When I installed a pressure tank for my well the bladder had to be charged. The unit came with a female pvc cap with 3/4" NPT that screwed on a tank fitting. The cap had been drilled and had a tubeless tire valve fitted to the hole. You could do a similar fitting to the center of the clean out plug.