I decided to redesign my dual Battleborn battery setup and went with an orientation that
barely fits. But it does, and I think it came out very nice!
The end of the box has two panels. The upper is for the breaker with intent that this panel won't be removed much. The bottom panel lets me get at my Victron battery monitor. I decided to mount the Victron panel display here for now, and left barely enough room for a Victron solar controller which I plan to get soon.
There's basically no room to fit on the sides, so I omitted sides! I hope to attach something plastic over the terminals, but for now I left it open. As it is oriented in the trailer, you can't get to the battery terminals except by slipping out the murphy bed access panel (which can be done).
The box has handles so I can heft it out at the end of season and bring inside for storage. That's really a two person job. It's a heavy bugger.