Best small woofer for my truck

I have 550 Watts and no where to go...literally. The last 2 channels on my Arc Audio KS 900.6 amplifier have yet to be filled. I can bridge the amplifier to 2 Ohms, but no less. I have this amplifier in my 2010 Toyota Tundra crewmax.

The only real place to put a subwoofer box is behind the seat in the back. Leg room is a premium for me, so I am looking for a box that is as thin as possible. One of those giant ported JL12W3 just isn't going to cut it.

I would like the best sound possible with the smallest box possible. I have even considered just doing it free air with a WS 10" sub like some of us have done in our boats. My current JL 10" sub rocks pretty well in my boat with this same amp hooked up to it. But I have never heard of a free air sub in anything but a boat. Do people do such a thing?

There is also a budget here. It does not need to be the loudest sub out there, but I am looking for the best bang for my buck in the littlest package for under $250. Any suggestions? I am also capable of building my own box. I can make it pretty wide, and narrow to get the appropriate volume of box if necessary.