I certainly love the pair of 10's I have in My Eclipse. I got the BA's partly on how great their Component set I have in the front is,a nd partly on that very same airspace requirement (I put mine into an enclosure sunk into the spare tire well).

There was a product called "Variovent" a few years ago, I used them and believe I had great success. I say believe, because it was about 10 years ago and I didn't have such a critical ear then. That, or some other form of "aperiodic membrane" may allow you to perhaps have a pair of 8" or some other configuration to get what you are looking for. Thinking quickly about some Bose speakers, or headphones for example, I wouldn't necessarily conclude that an 8" can't make good bass. Of course, it depends on what you're loking for and what music you listen to.

I'd check the manufacturers websites. Almost every company seems to have speaker dimensions and box sizes both on their websites ready for download. You may come across something that fits your requirements. Good luck though, the trend these days seems to be towards giant magnets.