Since you're sacrificing the compression tester anyway, why not use the female thread that fits the gauge, cut off the barbed fitting with the rubber cone, and silver solder a more conventional fitting onto the end? Sometimes the glaringly obvious solution to the problem gets ignored by engineering types who make the perfect solution the enemy of the "good" one. My younger brother with all the letters behind his name does that all the time!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!