My subwoofer system consists of:
Alpine SWR-1040D 10" subwoofers (x2)
JBL BP1200.1 Amplifier
Alumapro 5 Farad Capacitor

I want to get rid of the 2 10s (enclosure taking up too much space in my trunk) and go with one subwoofer. The new enclosure would be .6 cubic feet but I know fiberfill/polyfill can be added to the enclosure to 'increase' the volume of the box. Due to location, the subwoofer can be no more than 12". Any recommendations?

I'm also looking for trades, possibly for that subwoofer or other car audio/video gear.

Things that I have:
Several Alpine Type-R dual voice coil 10" subs (5, I believe)
Alpine 12 disk changer (CHA-1204)
Alpine CDA-7844 CD head unit (40 x 4)
Single 10" sub enclosure
Double 10" mirrored sub enclosure (currently in my car with 2 Alpines in it)
Triple 10" sub enclosure
Alpine CHA-1204 12 disk changer
Alpine MRV-F407

If you'd like to trade and you have pics, I have pics too.