Mattk11 has an awesome underseat design for subs, but requires custom fiberglass fab....
After a recommendation from another forum user, and reading the reviews I figured I couldn't lose for $170 for a
Rockford Fosgate P300-10 Punch Powered Loaded 10-Inch Subwoofer Enclosure
for my JKU. I haven't installed it yet, but plan to mount it right behind the narrow fold down seat with some sort of fastener that I could lock.
Cool thing about this is the remote control volume and the fact it easily unplugs to remove if necessary. The box seems well crafted and I also believe you can keep the existing sub working as well.