Here's my subwoofer box design. As I mentioned previously, I've gone with a push-pull design to reduce the size of the box. I'm also using two slot ports. The whole thing will be built out of 3/4" MDF and then finished with carpeting.
The drawing doesn't include the contours to make it fit in the back seat wells. I'm going to have to cut that to fit. The volume there won't be exact, but after playing with the numbers and looking at the frequency response curves, I think it will be within my threshold of satisfaction.