that box is much bigger than 0.8cu ft, it;s measurements alone put it nearer to 1.2cu ft (at a guess)
maybe the internal volume once the displacement is taken into account is 0.8, but in no way can a box measuring over 1ft square on all dimensions be under 1cu ft
i think, if you modelled it correctly, with the correct chambers in the correct places, you would look for around 2.5cu ft without taking into account the displacement of the sub