the majority of people like ported boxes because they are louder and go lower (if you design em right). I would just go with whatever ascendant recs.

good idea about taking out the 6x9s since they usually sound like hell anyway. You might want to think about sound dampening your trunk or making a better blow thru system (either through your 6x9holes or through something else like an arm rest etc.

I would go with 4 cubes net tuned to 28hz with at least 40 square inches of port.

Let us know your dimensions then we can design it, try to figure out the maximum size your box could be and still get in your trunk, look for all pinch points and take the smallest combinations of all these that way you don’t wind up with something that cant get in your car.

If you are anywhere near Spokane WA then I can work on the box for you, if not then shipping would be astronomical.