You could get a JL box it fits in that back left corner and has a 10w6. that should be very comprable sound to the polk probably better. If you are proficient with Fiberglass you could make your own. I am gettin ready to make my own for an Orion HCCA 10 sub :). you can also build a box that fits in the center and you can put your t-tops on top of it. that's what I have now. e-mail me if you have questions. I don't know price exactly but I would guess in the 200 range depending on the materials used.