1/2" all-thread (threaded rod) holds the box snug to the bottom of the bus body. We'll trim the excess all-thread below the box at a later time.
The box sits a bit forward in the hole due to the presence of a bumper support at the rear (just visible near the upper right of the box). Moving the box forward a few inches allowed us to maximize the depth of the box while avoiding building an odd-shaped structure.
1/2" hardware was also used to fasten the tank bracket to the bottom of the box.
We intentionally left a gap and the bottom and top of the rear of the box for ventilation.