Convertible Subwoofer Location

I'm looking for some options to increase the bass in my 68 convertible. My current set up is a RetroSound RC900C radio, 4-channel 50/w RMS amp and Pioneer 6.5 kick panel speakers. Highs and mids sound OK, but zero bass. My thought was to build a hump hugger subwoofer box that would fit behind the console hump hugger and put 1 6.5 sub there. Anyone else have something similar to this and how does it sound? Any other options for a convertible?

Iskwezm, thanks for the advise. I really like the idea of a self-powered sub and putting it behind the back seat. I took a look back there and there is not a whole lot of room. If I put it between the seatback bars, there is 11WX14HX4D. I looked through Amazon and didn't find anything with that small dimensions. Any advice on units you've used? Also, I still have the stock alternator. Do I need to upgrade that as well? This is great input. Thanks again.