Damn, that is really thin. I didn't even know subwoofers came that thin. This one caught my eye:
http://www.crutchfield.com/p_130SWX251/Pioneer-TS-SWX251.html?tp=112
They're going for around $120 on ebay. Good reviews on Amazon too.
This thing is exactly the same thing, and can come in a loaded enclosure as well. I just figured that since you are trying to fit it into a round hole, the "square peg" might be a problem. I'd try my hand at building a custom box to fit the well instead.
Funny that you're moving the battery back, I just moved mine forward. All because the conventional sub/amp enclosure I have behind the seat will have to go when I start putting the rear mount A/C system back there. AND like you, I've become so accustomed to having the bass now, I don't want to lose it.