i am going to purchase an Alpine SWR-T12 and it will be the only subwoofer i am limited on my enclosure since it is under my seat of a 2001 ford f150 Ford F-150 Supercrew Cab 01-03 Dual Subwoofer Box is the box but end of day i have had alpine type r's and have heard the shallow mount subs in ported and would prefer ported i realize this is more or less a prefab box but i would like to put a tube port in and i am confused on how to calculate the legnth and diameter if anyone could give me some advise i would apreciate it