I saw suggestions from others about installing it to the headliner, but there was concern about how solid it would be when pushing buttons. I gave it a go with some sticky-back heavy duty velcro and a few dabs of GOOP adhesive for extra hold. I found the sticky-back stuff may peel off plastic if the temperature gets too cold.
I think the trick to making it solid is to "pinch" it in place with velcro from two different angles. The velcro on the top of the gauge holds it in place, while the rear velcro, at almost 90-degrees, stops it from swaying when pushing, or pulling on it. The cable also adds some stiffness. It can take quite a poke now!
I also ran a 6' straight-through Cat 5e extension cord through the headliner down the A-pillar and connected it to the OBDII cable. It gives me the option to move it to another car if I need to.
I'm still looking for some heavy duty velcro in grey to blend in with the headliner cover better...