Whoa, crazy I just listed something on ebay you might be interested in!!
I was just getting on this part of the forum to put a link to my thread in the JK section, when I saw your thread here. Check out my thread and the ebay auction.
I didn't want to put anything under the seat because that ruins cargo space in the event of needing to fold down the seat.
Here's a link to my thread
I built this with myself in mind. I seriously doubt that you could find better quality anywhere else. Check out reviews on the sub. It top of the line, and according to all reviews it hits hard when cranked up. I've only tested it at low power because it's not broken in yet. The sub is a JL Audio 8w3v3-4.
And here's a link to the ebay auction.
JL Audio 8" 8W3V3 4 Custom Jeep Wrangler 4DR Sub Enclosure w Sub | eBay