I would reccommend what I am going to do at some point(when i get the funds).
Im going to create a sub box that will fit in the spare tire and use either a 8" or slim 10" sub. There are a few options to place your amp, im still undecided on mine.
If you have the time and the means, you might want to consider doing something like what listed below.
Did-it-myself custom box for 370z (56k beware)
10" sub in spare tire!