I drill all four corners with about a 1/2 inch bit. Score a line around the
perimeter of the box with a utility knife. Then use a sawzall, with a 6"
blade, fairly coarse. The key is high speed, but low pressure on the blade.
Otherwise, the lath will be shaken loose. After opening up a hole, I can
reach in and hold the next lath. Also, cut the side closest to the stud
last, if possible. Different houses, with different plaster, will cut
differently, of course, but this method works for me.