Sorry it took so long to get back but I went ahead with all your suggestions and stripped my entire trunk to find my absolute max dimensions and came up with this:
Height=16.5", width 32" and depth I want to stay behind the trunk lid opening, (if that makes sense), so depth 20".
The box is 3.35 cubes, 52.5sq" port area, 15.66sq" of port area/foot, tuned to 33.44hz
What ya think?