Nice bit of kit and the lockbox is a nice touch. I didn't see INSIDE the box, but how is it fastened to the unibody of the SUV? I could only see bungee cords pulling the box forward into the rear seat.

I presume you are travelling at normal highway speeds with all that heavy equipment back there rather than rock-crawling slow. Having experienced quite a few off-road bumps, bungee cords ("eye excavators") cannot reliably prevent loads from shifting under minor shocks. Ratchet straps crossing over and attached to loops bolted to the unibody are better.

The best solution would be large fender washers, nuts and bolts through the box and unibody. I would get at least grade 5 if not grade 8 bolts for the shear strength.