Small Car Sub page.
This sub uses two MCM 55-1170 6.5" woofers. The woofers are 8 ohm poly cone drivers. I use two in each sub mounted in a 6" sewer pipe. The PVC pipe and end caps totaled about $8, and the woofers are $9.00 or so. The whole contraption measures just over 14" long, very small for the amount of sound you get.
The end caps I got were very flat, with only slightly rounded edges. The woofers are kepted about 1/4" apart by a section of plastic tubing, normally used for fish tanks. Epoxy the tubing all the way around the woofer gasket. Use mortite to seal everything. Here is a drawing on how I mounted the woofers.
Right now I've got two of these subs in my truck. With a single driver mounted in the ~.125 cubic foot enclosure the PVC pipe creates, the F3 of the system is 80 Hz. With two drivers the F3 is 60 Hz. The system has less than 1dB of ripple, so the sound is very tight. Two, with a 35W X2 amplifier, crossed over at 80Hz/12dB are capable of producing 110 dB SPL in my truck. Not bad for less than $200.