This is a gauge mount for the AEM 30-0300 AFR gauge, although it should work with any 2-1/16" gauge. It was designed to go on my 1978 Chevy C10 on the a-pillar. The gauge ring is adjustable so you can angle it based on your setup. It requires 1 3mm bolt about 20mm long, but you might have to file that a bit.
It has three parts: The base, the ring, and the gauge retention bar. Bolt the ring to the base with the 3mm bolt (it will cut its own threads) but don't torque it down yet. Dry fit it on your a-pillar and adjust it to the angle you want. Then torque down the bolt. Mark the a-pillar where you want the base to mount and drill your holes. Screw in the base and install the gauge using the retention bar.
Prusa i3 Bear Upgrade V2.0 (sourced clone)