The batteries are installed and all is well.
First thing was to remove a few raised bits on the bottoms of the APC cases.
Next I needed to make a wall to keep the left batteries in place. I used some wood trim I had left over from the kitchen cabinets.
Then it was finish up the wiring. I used wire nuts so the batteries can be easily replaced.
Here it is all together. I’m thinking of leaving the battery charging cable loose so it can be pulled out when needed. The alternative of fixing it seems more problematic at this stage.