I think the subzone box is the brand that Louisw is using in his car. One advantage is that the angle on the box he has fits perfectly against the back seat.
Always pick the larger enclosure if there isn't one that is perfect (this only applies to sealed enclosures, ported enclosures need to be perfect). Actually, the subzone box probably is perfect. You can usually count on a 10 or 12" sub to take up about 10% of the box volume which would result in .78 cu ft for the subzone box. Check the sub mfg recommendation on box size to see if it already recommends using polyfill.
Polyfill will allow you to undersize a box by about 15%. The problem with an undersize box is that it builds up more backpressure which in turn heats up the voice coil. Hot voice coils result in dynamic compression which will cut your maximum output, sometimes pretty substantially.
The other bad result of a small box is a higher Q. The higher the Q, the sloppier and boomier the bass.
