I also worked on the under-seat batteries. First, I cut, drilled, and painted a hold-down bar (made of 3/16" bar stock). I also installed threaded rod to hold it down (and a 3/8-16 RivNut in the middle). One note on the threaded rod - it actually goes through the 1/8" angle stock and through the Jeep's sheet metal. Unfortunately, there was not a good flat spot to attach the 3/8" rod - at least, no place where it would stick straight up. So, I just bent it - bending it once should not weaken it (repeated bending would, of course). Here it is:
I cut tops from polypropylene, and drilled holes for the conduit between the two boxes:
Here it is, very nearly complete, with the conduit between the two batteries installed. You can also see the connector for the conduit from the rear rack at the top of the picture, and the connector for the conduit to the engine compartment at the bottom of the picture:
One note - always consult your plans as you cut. You might note that there are *two* cutouts on the box at the bottom of the picture - I had put the batteries in backwards, and cut the openings based on the incorrect orientation of the batteries. As I consulted my plans, I discovered my error, and cut new openings. Not a big deal - but - measure twice, cut once continues to be true...