Today I went over to Lane Aviation to buy some nutplates. Wouldn't you know it, they were out of stock of the single lug ones. Well my friend, Greg, was nice enough to let me borrow one of his, so I was able to get this last nutplate installed.

The next task before riveting on all these stiffeners is going to be countersinking some of the holes to accept CS4 pop rivets. This had the potential to be a tricky task since some of the holes are between stiffeners and are also only 3/8 inch from the back face of the firewall. What I decided to do was make a fixed length microstop from a piece of brass tubing. It took several iterations to get the length exactly right, but here it is.

It just fits over the countersink bit as screwed into my right angle drill

When the bit stops countersinking, you're done. Works great on scrap pieces so far. Hopefully it'll work just as well on the actual stiffeners.