I decided I didn't want to have to loosen a billion screws and take up an entire floor panel just to service the aux battery I mounted under the baggage floor, so I measured out a small cover plate that will make access a little easier.
I cut out the corners with a hole saw:
Then with snips and files I finished cutting out the hole in the floor. Here it is being test-fitted… I can get the aux battery in and out through here with no problems.
I made a cover plate out of some scrap 0.032" and match-drilled and dimpled it to fit the existing screw holes. It's plenty strong, and now I have less than a dozen screws to undo to get at the aux battery.
The cover plate isn't flush with the floor, but since this area will have carpet over it you'll never know the difference. I did use my edge roller on the plate before I dimpled it, which helps it sit down nicely against the floor.