This is what the airless caps look like.
Six Robblees has these, Schrader Brand in this case. They had the Borg valves on hand in Anchorage for our 8 "hand hole" (those tiny ones!) wheels and could get others.
I got these at Six Robblees
for the other three wheels (Fronts, Spare) and they weren't long enough to clear the center of the rim for a gauge, but it works with those airless caps.
Beware with shops that deal in larger trucks. The common valve hole size for them is .625" and our light truck size (same as cars) is .453". Some valves are adaptable to either size.