Old school alignment guys; gauge mount idea. What am I missing?

I just use a Fastraxx gauge, it clamps onto the outer lip of the rim. You have to make sure it's tight to the wheel, but it works well once you practice a bit with how much tension to put on the gauge to hold it to the wheel. They make a version for 13-17" wheel and 17-22" wheels.

SPC Performance 91000 SPC Performance FasTrax Camber Caster Gauges | Summit Racing