Unexpected project while swapping an ugly ceiling fan for a new light fixture. It's really hard to find a nice looking flush/semiflush light fixture unless you like old-fashioned stuff, but I digress.
Found out that pan box stuck out too far from drywall, and couldn't mount the light base flush. Swore a lot.
After a few hours of running around, eventually found this amazing polycarbonate old-work box at the Mooresville Lowes, they just started carrying them (no one else in the county does yet):
It only comes with two screws, not three, but doesn't matter.
Traced, cut hole, mounted box.
Patched hole.
One edge is at a taped jointed, which is why it's below surface.
V cut all the joints, overlay with mesh tape, mix up some 30 minute setting compound, slap it on there and let 'er go until the morning. The deep joints shrank up, but that's what the next coats are for.
I really hate patch work. And painting over patch work.
FYI, they also make a fan-rated version of that box with 3 lag screws, and it has a metal side bracket that wraps from the stud mounting face to the fan studs. It's probably the best fan mounting box I've ever seen: