You get a high amount of cabin gain in a car and that gain will flatten out a sealed sub pretty well. It would likely do the same for a bandpass box but why go to the extra trouble? All the bandpass will do will in return for the extra complexity is give you some roll off on the high end - which isn't worth much.
BTW I have a 2010 BMW 535i with a 12" Earthquake low profile sub playing through the ski hole. Its a sealed box just over 1 cu.ft. with a slant front that matches the slope of the seat back. The driver is recessed to leave room for the surround at full extension. It does quite well. The most obvious benefit is eliminating the boom from the stock premium audio setup that you hear even when the program isn't bass heavy but on bass heavy material it makes itself felt.
If I were doing this over again I think I would go .with a IB type sub playing through the ski hole. 12" AE IB12AU would be ideal. The high Q of your Sundown driver suggests it might work well IB also. Just some how seal a baffle/mainfold against the ski hole and use the trunk for the enclosure.