Updated photo showing a cleaner wiring design that leaves the batteries accessible for checking and adding distilled water as needed.
Also switched to those flange nuts for the bolt terminals that Lewster recommended. Much better, as it's not possible to get wing nuts tight enough to secure big cables.
I added a flange to help the retainer bolt hold the batteries in place while distributing the pressure across more than just a bit of the shoulder of each battery. Looks better to me now.
I imagine one could install the batteries with terminals more to the center if desired. This orientation, however, allowed me to use the existing house wires without inventing any new 4-letter words. (The length of a couple of the cables was quite limited.)
I experienced zero risk of, or actual, arcing of any kind when dropping the batteries into the case as shown. Also none while wiring, as I used due care.
If you're not comfortable doing this, you are free to take another approach. No need to armchair quarterback. Also, please remember a photo is always a 2D rendering of a 3D environment and can be deceiving.