i just measured my trunk and the usable space is 36" wide 16" high and 26" deep. this will be the first box ive ever built. is that enough room for a good box for a 12?
just make sure you can fit it in once that is made. I have seen many people measure out their trunk, and build a box to fit then can't get it in the trunk once it is built because the trunk opening is smaller then the trunk itself. That is plenty of room to run a 12 sealed or ported.