Only way I think it would fit behind the seat is if you remove some of the plastic back there and mount it in the opening. There is just over 5" of space inside the plastic area. Under the seat, yes.
I think I might put it behind the back seat. I measured all the way at the bottom and theres about 7 inches and if i cut some of the plastic away at a certain spot the foam behind it goes in about another inch. I think i might be okay with this and just mount the box there.