I'm planning on adding a lot double-gang old-work boxes to a wall that is made of 2" foam and 1/2" drywall on top of 2x4 studs. The tabs on old-work boxes don't reach far enough to grip the back of the foam, so I need to make some clearance right behind the drywall in the upper right and lower left corners so it can grip just the drywall. What might be a good tool or method for removing a chunk of foam that is set behind 1/2" of drywall? The chunk removed needs to encompass the "swing" of the tab, and be at least as thick or slightly thicker than the depth of the tab. The drywall needs to remain intact for the tabs to grip. I'll be doing at least 20 of these boxes, so I'm less concerned about precision and am more interested in just getting it done. Even if that means removing the entire thickness of the foam in that spot, the spaces between the bays are insulated yet.

Any ideas are appreciated!