I cut in first with my 4" diamond blade to make the outline about 1/2" deep, then I use my 2" coring bit on my rotary hammer to remove most of the material, finishing it off with a chisel. If the box is outdoors I will get some mortar to set it, but if indoors I will just mix up some 20 minute joint compound since I keep that on the truck...it holds like you wouldn't believe.
