Okay, here is the thread on my install,
Clicky. The CV's came out when I got a 3rd RSDc, I bought the CV's for a special project, and they are now back in their original boxes.
Anyhow, there are specs on the box I built in the link above. It was a total of 6.2ft^3 net internal volume and tuned for 28Hz. I built it with RSDc's in mind, but I modeled it with the strokers in it and it was a good fit for them as well, so I rolled with them while I waited for a third RSDc to show up at a decent price.
I can come up with box specs for you for something similar with one 12D2, but just because I liked it doesn't mean you will. You'd be looking at a box with a net volume of 2.07ft^3 for one sub, so it is a fair amount smaller. Tuning this thing low is what makes the biggest difference SQ wise. I put one one in a box tuned to 45Hz like the manual suggests, and it was not what I was looking for.
As mentioned above, it would be great to see how your amp is set up too. I don't know how much you should really expect out of one single sub-- not if SPL is what you have in mind anyhow. I ran a single stroker 12D2 in my extended cab pickup truck for a while, squeezed the box under the back seat

It was plenty of sub with one Ti800.1 driving it, but there is a lot less cabin space in both my Jetta and the P/U truck.
For what its worth, when I had the 3 strokers in the Jetta was beyond insane. It was my loudest install yet, and if the enclosure is tuned right those OS strokers actually have amazing SQ. The stroker is the only ported sub I have ever encountered that sounds as "tight" as a driver in a sealed enclosure. I'm no where near as happy with the sound of the RSDc's in the same enclosure. They are pretty ridiculous down low, but they really get sloppy above 45Hz or so. Their specs really are optimized for sealed enclosure, and when I finally find time, that is what is going to happen to the things. I'll gain back a lot of install space-- er... I mean trunk space, too
Later,
Jason