I have to agree with the above, the enclosure is pretty cool-looking, but it's going to mostly rattle the trunklid -- i see that even the ports are to the sides so it's not going to send much bass to the passenger compartment. I prefer a sealed enclosure myself, you can get a pretty decent-sized square box into the trunk and have some more room on the sides. Benefit is you can move it to a non-Mustang at some point and it will still work.
I bought mine so long ago I have no idea what the specs were. Mocking it up with some cardboard is a good way to go and pick a generic enclosure that matches your size-specs, make sure you can get it in without bending it.