Drill hole to mount metal junction box

Question

I am trying to install a metal junction box behind existing drywall. I cut a hole in the drywall next to the stud, and want to screw the left side of the box into the stud, but the holes that are pre-drilled into the box are too far forward and would go into the drywall, not the stud. Pardon the crude drawing.

Ideally, there would be screw holes further back in the box so I could install it into the stud, but I can't find any metal boxes that have that. A few related Diy.SE posts suggest I can just drill my own holes (Can I side-mount metal box with holes in the back?, Why not use a plastic receptacle box for an outdoor outlet?) but doesn't making my own modifications violate the UL listing of the box and thus electric code? Am I allowed to drill my own holes (with some concrete information I can show my inspector to justify that), or if not, is there another solution to mount a metal box behind drywall?

Edit:The purpose of this box is to connect wires that run behind the drywall to conduit that will run on the surface. Thus, following a suggestion from the question in Best way to bring wire from flush-mount panel to surface-mount conduit I was going to use an extension box. If this is an X-Y problem, or if there is a better box or approach to doing this, I would appreciate suggestions.

Answer 1

That box should have a mud ring of the correct thickness to account for the drywall and/or other wall finish mounted to the front to mount devices to. At which point the holes it has should land in the stud.

Image source - example not endorsement.

You could also use a box extension ring if you really just want the blank cover that fits the box type rather than a device or a finished cover plate that needs device mounting holes.

Answer 2

Get one like this one Raco box with bracket from Home Depot. It gives you the space and holes you need to mount it to the studs.