I used to own a box like this with a 10" pioneer sub. It pounded harder than two 12".
So the box is a completely closed off unit, only the two ports to let air out, I have included a picture of what I can remember it to be, please tell me if you know a program I can use to design a box like this one again.

 

winisd at linearteam.dk, do not download the pro version...that should help you out.

 

That looks like a vented bandpass box. Some people would call it a 6th Order bandpass. VERY tricky to design and they sound VERY bad when designed or built improperly. If you have little to no enclosure design experience, it would be a good idea to seek help.

If you post your sub and specs here someone may help you out.

 

That is exactly it Dukk! I am talking to a member here about purchasing a 15" Alpine type R I have built one Box before, for a 12" MTX Subwoofer, I downloaded the plans from their website. For my first attempt, the box turned out pretty good, My Uncle is still banging it in his eighteen wheeler two years later, and he won't part with it although he's been offered 200 cash for the empty box. I appreciate any help I can get designing this box. I'm kind of scared that the Box won't fit my trunk (1993 Pontiac Bonneville) Beacause I know that the box for the 10" was pretty large.