My woofer is free air which everyone said is bad. A friend of mine also with an SVT went with the JL Audio 8" enclosure and hung it from the rear deck. He had to add some support, but the spaker itself faces the back of the rear seat. We went differnt routes with the sub idea, both wanting to minimize trunk intrusion. As for the bass my friend was impressed and it doesn't sound boomy either. He did get a better amp with built in crossover for doors/sub. I'm just using caps & an inductor for now until I get the right freqency. Then I will get a 3rd order crossover.
As for cutting another hole in the deck, it wouldn't work for my 2000 because of the middle shoulder belt tensioner. For me, however, I think 2 of my woofers would not add to my environment, but I don't want a bass heavy system. I'd like to in the Spring check it with a sound meter. I leave the heavy bass to my home theater

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Your idea sound unique and should tak up too much space. You will have to add supports because the metal is very thin. I don't know how much bass you want, but you could try mouting just one in the existing hole, hook it up to the amp you'd use and take a listen. Just set the amp in the trunk and listen to it. If that isn't enough, then go one with your plans.
I don't know much about those speakers because I generaly don't buy into main stream equipment, but you can check out mine at
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Product_ID=7760&CATID=49. I'm new to the forum posting so I don't know how to do the link. Just copy & paste.
I've gotten quite a bit of stuff for both my home and car audio from Parts express. A lot of DIY stuff.
I'm from the Detroit area. If you're close, maybe I can show you how I did it.