RE: Infinite Baffle
If you're concerned about trunk space, and not so much about bass, why not go with a single speaker? My single 12" solobaric has gotten my lincoln to 133 dB, and it doesn't take up much room. It would take up even less room if speaker were mounted in the rear deck and the box hung down below it. But you don't want to run a solobaric without a box.
I personally wouldn't do a ported box, because i'd rather have sound quality than volume. A ported box is going to send some conflicting sound waves out of those ports, and is going to muddle some frequencies. An infinite baffle is trying to do the exact opposite, trying to completely isolate the front sound wave from the wave coming off the back of the speaker (since they're opposite and all). An sealed box isolates the two sound waves, but requires more power for the same volume because you have pressure changes inside the box to deal with.