Today, I finished drilling the screw holes in the cover plate and attachment ring to #19 size, then drilled the ring for the nutplates. Next, I countersunk all the #30 holes in the attachment ring.
Then, it was time to make the big cut -- the hole in the fuselage. I simply laid on the floor next to the fuselage with a Dremel tool and trimmed close to the lines. As always, it feels odd to hack a big hole in a nice, perfect skin. Once I'd cut a rough hole, I used a notching tool, a 2" drum-sander, and various files and sandpaper to finish off the edges of the hole.
I dimpled the cover plate and attachment ring, and as I expected, they both curled quite significantly in the process. I spent about 30 minutes flattening them by hand. (I've found this to be the best way to do this. There is no tool or process I know of that allows the required level of finesse). I clecoed the attachment ring to the fuselage skin and fine-tuned the fit of the cover plate. It looks really good!
Finally, I primed the attachment ring, the inside of the fuselage skin along the line of rivet holes, and the inside of the cover plate.