I think relocating the battery is a good idea, and some day I will. I don't understand why Toyota put every damn heavy thing on one side of the truck. When you put duals there the problem gets even worse, not to mention the fact that changing a headlight bulb becomes a PITA. The best place for the batteries would the front right corner of the bed. I wish you had panned out so we could see how you arranged things back there. That box looks pretty big. I take it you have other things to go in there eventually? Is there ventilation through the box? Things like the isolator need ventilation. Everything looks cleanly done.