This evening, I installed the battery backup in it's new home. The left nut plate ended up being frustrating. It ends up being sandwiched between the sub panel support. Holding it in place for riveting was a challenge. I ended up using my magnetic finger to hold it while I got the first rivet in place. Then, I was able to twist it into place.
I think I like the new arrangement of equipment BETTER than the old one. It puts the fuse panel in a place that's easy to get at.