To my knowledge most sub speakers are "free to air" or should be. It would have to be a very large enclosure if not! Most bass boxes are ported and therefore act virtually the same as free to air.
The very best enclosure type for a bass speaker is a Transmission Line. This is used in to of the range Hi-fi and is basically a tuned horn shape which is ported at the small end, speaker at the big end pointing forward. I have sort of made this in my other car.
I take on board what you say about keeping it out of site - but i intend to make this stand out. In my experience if someone wants to break into your car they will and hiding it away is no deterrent, they can hear it when you turn it on!
My other car was in a locked garage, locked and alarmed. They broke in and stripped the stereo. Thats headunit 6x9's in the doors 7x10's in rear shelf 12" sub and 4 ch amp in boot!
If ya got it flaunt it i say!