After HOURS of sanding I have one pickup ring. I am using 60 grit, which is usually pretty aggressive, but man, this is slow! I did some of my test cuts with 1/4" balsa wood, and was surprised how "test" wood ;-)
Sitting and sanding, mostly I thought about how there must be a better way to do this. Best idea so far would be to use double-sticky tape to attach the piece to the bottom of my palm sander. I could then attach sandpaper to the top of the guitar, with low tack tape like I am using now. Then I could hold the sander with the attached piece down onto the sandpaper.
...I'm not going to do this for the remaining piece because I am afraid that a power tool could way to easily mess up the top. But the longer I sand, the more tempting the idea...