Another option mimics the factory sub woofer installation in the wife's Porsche Cayenne turbo.
They put it in the floor, in the spare tire well.
Hers doesn't have the optional spare tire, so the sub woofer fills the entire well.
Those w/ the spare have a smaller sub woofer that is then straddled by the spare tire / wheel.
The sound from this location is great as is the utility from this location.
2011 GT Mustang - Performance White / Brick Red - Cashmere : 401A : Brembo : Electronics PKG : Spoiler Delete : 3.73:1 : Glass Roof : Comfort PKG : Security PKG : HID