I installed the anchor nuts on the two backing plates. The rivets are countersunk so the holes have to be recessed.
I used these anchor nuts INSTEAD of those called for by Sonex: - MS21078-3, Two Lug Anchor Nuts
The type called for by Sonex have two major problems: 1. They are easily stripped by the anchor screws. I know from experience. 2. The screws can back out.
These alternate anchor nuts have the plastic lock-nut inserts to prevent both issues. Use washer-head screws (photo 2) in these. I had to enlarge the holes in the backing plates slightly to make sure the screws would fit.
