it isn't as much about sound as it is about having the correct enclosure size... there are programs that can help with this, and I am sure that a box could be built into the sides that would accomplish this.
however, I wouldn't just do it without finding out / purchasing subs for your specific application. I think you can get subs that don't require a box. I'd get a crutchfield catalog, and although I don't buy from them (prices WAY to high) they have great info in thier catalogs that would basically teach you all this / give you the various option.
<font color=blue> Jeff - may the force be with you</font color=blue>
<a target="_blank" href=http://coloradok5.com/gallery/Jeffs-Stuff>link to k5's</a>