The box is constructed of 5/8" MDF and 1/4" plywood, shown here on my sophisticated painting platform.
It contains two 400 amp fuses and the BMS Control Unit assembly along with terminals for the cable run from the rear pack to the front pack and controller. Each battery segment of six four-packs is isolated by one of the fuses.
The negative terminal will also act as the negative connection for the DC/DC Converter and the Charger.
Gland nuts provide the entry point for the 2/0 cables, consolidating the rear pack wiring into a single hub.