OK, I have a 97 GT Convertible and I just purchased a 15" Polk Momo Sub. Now my delima is where to put it. I don't want to take out the back seat, so please no one propose anything like that. The big issues are security and quality. I really don't want the sub displayed so that every kid passing by gets the idea to rip open my top and take the sub. But I also want to get the best sound quality possible (I know its a convertible, so it's never gonna be that awesome with all the wind noise and crap, but at least to the point where I can make good use of my investment).
So I'm open to suggestions. If you have any, please post it here.

You'll probably get pretty good sound if you have the sub in your trunk, because with a fifteen, it's either the trunk or your backseat. Had you purchased a ten or a twelve, you may have been able to remove your rear speakers, build a custom fiberglass enclosure where the rear speaker used to be on one side, and put a sub in there. Also, it may have been possible to use a small sub on your rear deck, covered by mesh, though I'm not exactly sure how a rear deck is setup on a convertible.

I'd just suggest sound deadening your trunk, and then building an enclosure for the trunk. In my trunk, I've found that I got the best sound when I had the subwoofer aimed at the rear of my trunk.