Use a pop bottle cap, drill 1/2 hole in it, and connect corny gas post to barbed part i bought at lowes for 2.50.
I have tested it up to 120 psi for 24 hrs and no leaks.
Carbonator caps are good for re or over carbonating water, oj, lime juice and beer.
120psi lemon water tastes like alkaseltzer.
Here are some pics of the parts:
If you plan to test high levels of pressure, be sure to wear safety glasses.