When I have to work in lathe and plaster, as in installing an old work box, no matter how I cut it, I try to find the best way to wind up with both ends of the box on a lathe, otherwise it seems like I can't get the box or device anchored securely. I've begun to shy away from powered saws for this as it seems to do more damage than just going slowly with hand tools. Sometimes just drilling a bunch of holes around the outline of the box and filing with a sheet rock file works best.
