Just throwing this out there if you are not trying to build something to specifically integrate with the factory audio system...
I have this JL Audio micro-sub enclosure in my Mirage: CP208LG-W3v3
It fits fine in the back of my hatch. There's lots of information online about the speakers used in this enclosure (8" W3v3). I don't know what your budget is, but I am not disappointed in how these perform. Even the version with a single 8" sub (CP108LG-W3v3) would satisfy most people. The dual version was on sale so that's why I bought it.