look at your manuf. suggested air space...some of you guys have extremely huge enclosures for 1 or 2 subs....for example, i have 1 Polk MOMO 12" (600w PPI6000 @ 4ohms) in a .89cbft box that i built into the side of my trunk, where it bulges out...theres way more than 1.3cbft in those pockets and you can build something in there fairly easy and create a flush wall without taking up much trunk space at all....**sidenote**if you match up the size of the box to the sub in question, you will get a much cleaner sound....also, if you plan on building a box, and your good with MDF, dont just make a square box, put some angles into it, this will make the accoustics cleaner....