I actually built the entire box, then slid it into place. The bulk head is only there to hold it in position. Behind the actual bulk head is 3/4 inch particle board, which is what the entire box is made out of. I slid the box into place, then cut out the bulk head so the speakers would be in that spot. I ran a bunch of self tapping screws through the top of the bulk head and into the box to lock it into place.
The box is massive, but it doesn't take up any leg room out back. I made the box follow the angle of the rear shock hoops and rear wheel wells, and I had already moved the seat way back to the tail-gate. I can actually fit a huge tool box between the rear seat and the box, and being 6'4", I have sat back there a couple of times and still had tons of leg room.