Screw a mounting plank in to give you something to screw the filler piece into.
Discussion
Def ain't pretty at this stage. And the plywood I used was way too hard for this purpose. Had a helluva time getting the screws in deep enough.
Mud the edges then tape over them with this mesh tape.
Two lessons learned so far:
1. I should have mixed the mud with a bit more water.
2. 20min mud wasn't a great call for a first timer who doesn't know what he's doing. I had to really race to patch the second cutout. I wish I'd bought more like 45min.
