^^side firing ports for the good sounding win.
which BTL? this year's or last year's model? rule of thumb with the ports, aim for 7 - 11 sqin of port per cubic foot of box.
don't be scurred of the calculations, though. for internal volume, all it is, is length times width times height, then divided by 1728. shazzamn, you've got volume in cuft.
Get in the vehicle, take some dimensions, and give us more to work with in terms of "how much room is he willing to give up"