Most subs need atleast a one square foot box to function correctly. I bought a Kicker Solo-baric 10" sub for my coupe because I believe it only needs a min of .6 sq feet, plus they are awesome subs. Very pricey, but cheap on eBay! Another consideration - I used to have to Infinity Perfect 12's powered by Phoenix Gold, I loved them, they were extremely loud and tight and comp level. One Kicker solo-baric 12 in a ported box powered by PG class D mono amp is louder. So one 10 inch kicker in ported box or just regular box would be plenty loud and very space saving!

 

I just installed 2 10" MTX subwoofers in a simple box for each and they fit snug in the trunk, although you'll loose trunk space.
Good luck

 

We make some boxes for the trunk as well. You can check them out here

-JustICE

 

Most subs require a full squart foot of air space. Kicker Solo-baric 10" subs are extremely powerful and require something like .6 sq. ft. You could install 2 of these without removing the spare, just need to have a box made.