I exchanged emails today with Rod Birch at Car Audio Innovations. I mis-read the website. According to Rob, they do not offer a subwoofer that sits behind the 986 seats. My apologies for the bad information.
Here's the link I was looking at.
Porsche Audio.
Once the page opens, scroll down and find the following link on the left side of the page: "Our Sub Enclosures for the 1999-present Porsche 911 (996) and Boxster (986)".
Click on the link and when the new page opens, you'll see the 996 solution I thought applied to the boxster. It's about 3/4 down the pages. The side-by-side unit uses 8" woofers.