I've got Boston Acoustics in my car: http://www.bostonacoustics.com/newproduct/ProSubs.pdf . I love them. They fit nicely in my 3G Eclipse spare tire well... Their indicated depth says they fit yours as well, and they have a small sealed box requirement.

Some random throughts:
1) How much power do you expect to put through them, and for how long at a time? You're worried about a 300W rating?
2) Weight is your enemy for performance. I ditched my spare tire so I could get a pair of 10's into my spare tire well, and even with a ridiculously strong fiberglass box, 2 speakers, and 2 amps, I still came out ahead in weight savings by removing that tire and rim.
3) Ditch the spare, get a can of fix-a-flat, mold a space for it in your setup, and get the sub that you really want - so you're not kicking yourself later, every time you listen to it or show someone else.
4) I've heard Diamond components, and read a ton of reviews about them. If one of their subs sounded bad I'd think it's much more likely to have had a manufacturing defect, been damaged, or been improperly installed (box size, box type, loosely mounted enclosure) rather than simply be a bad speaker design. If you really liked them otherwise, try to find another example to listen to before disregarding them alltogether.