Attaching Air Compressor To Junction Box
I'm not sure for a specfically "2-gang" box. These are made for 4" and 4-11/16 and octagon boxes:



and there are 1-gang "handy-box" covers with a knock-out as well.

A 4" box will take 2 devices, but some 4" boxes are set up with tabs for the devices, while others are set up with corner screws like this plate uses and need a cover to hold devices in place.