you should have plenty of room to fit that sub. If you build a box that is 13.25 in tall with a bottom that is 10 in, and a top that is 6.75 in you will have plenty of room. as far as length to stay covered by the seat you can be no wider than 48 in. all of these measurements are with no carpet, or plastic in the back, so if there is carpet or plastic your measurements will be a little different.
MTX has technical info on their sight that tells you recommended box sizes. Good luck and if you need help post up.