In my old 94 SC2, I recall there only being about 12" between the bottom seatbacks and the beginning of the spare tire cover. Because the seatback slopes into the trunk, there's a maximum of 9" at the top. This leaves a pretty small box, something that a pair of 10's would work in a sealed type box, but no more than that. Defintely not ported.
I haven't run a spare for close to 2 years now, and have never needed it either. And I'ev put on at least 40k in that time too, if not more.