I'm selling my 05 STI so my underseat sub/enclosure is for sale. You can read about the specifics here:

http://forums.nasioc.com/forums/showthread.php?t=825886

Basically it's a lightweight fiberglass enclosure. Installation was super easy (even for a person like me who's not mechanically inclined) and I was very happy with the performance. I was driving it with about 200 watts and it provided plenty of bass for me. I initially had a 12" sub in my trunk and I was MUCH happier with the underseat setup. I haven't come across another enclosure for the STI that provided this level of quality. I've had it for about 9 months. I filled in the mounting holes with hot glue and re-glued part of the surround for a more air-tight seal. This sealed enclosure doesn't leak at all.

$390 + shipping

I'm located in San Mateo, CA if anybody local is interested in picking it up and saving on shipping.

 

Man, this looks hot. But I saw the original thread, 599 is a lot of dough. I wish I even had 390 right now, lol.

 

Yeah, it was expensive... it was the only quality in-car enclosure I could find though. I like the fact that it's light-weight too, I used an older 12" enclosure in my trunk while I was waiting for this to be built and the difference in weight between that larger MDF enclosure and this little fiberglass enclosure is pretty staggering.