Believe it or not, they actually make great ones for the firebird and the firebird trunk itself is great for booming, granted I'm not much into that scene, but my brother went on about how great my 88 formula would be if I had a box in the back. Anyways, that big hole in the trunk is your best bet, you may need to have somone custom make it for you though, and I doubt you'll need anything more than 2 10's, it's not about the size and how many speakers you have, it's about how the air flows. I used to have a friend that used physics to design speaker boxes, he would do competitions, and with one custom box with 2 12's, he beat a guy who had over 40 speakers in a van, and spent nearly 20,000 putting speakers and subs and amps, and my friend beat him and not just barely, I mean BEAT him.