So, back in the 80's, I was at a car stereo shop and they were finishing up a custom box for a Porsche 914. Like us, not much space for a subwoofer. However, this place put the subwoofer in a shallow box in the front trunk and vented it into the cabin. It was very loud and hit hard. Downsides of this are you loose a little bit of the front trunk, and there is a 3" hole into the cabin as well. However, you get to keep the spare tire.
Just another option I have seen
I also used to work for a car stereo shop back in the late 80's, and did a number of panels for doors with MDF. Spacing it out a bit to squeeze in a 6.5" mid. I am sure with work, as Dr. Jeff is saying, you could have a good system and the looks don't detract.
Good luck!