Honestly, I think you should sell the one 12" you have, and then buy two 10"s. That will fix both your space/port area problem, and your impedance problem. If you are determined to stick with 12"s, you are going to have to make design compromises somewhere. Ideally you should use at least 80 sq in of port area, with 3 cubes of net airspace, tuned to the low 30's. Since you don't have space for that, you are going to need to sacrifice somewhere and it is going to cost you performance one way or another. First thing I suggest is you don't use a triple baffle, a double baffle will be fine. Since the box is narrow that actually makes it stronger. Getting rid of that third baffle should gain you some space.
Space isn't an issue I can go wider to 15" and a hair taller but then I'm over 4 cubes and those subs only want 3-3.5 cubes for the pair. My problem is the shape of the box wont let me fit 2 6" aeros. If I stretch it a little I can fit 3 4"aeros. That might be my best ccompromise to staying under 3.5 cubes but thats still only 37 inches of port area. Im sure I can make this work somehow, I wont be competing at all just want to turn some heads.