Option-
Buy a dual 12 inch sealed pre fab enclosure for your car, mount a baffle over the second hole to seal it off, just take a round peice of MDF big enough to cover the hole, line the outside edge with silicone and mount the baffle. Spray paint the baffle a color of your choice.
If the box has seperate chambers internally, take a drill with a hole saw attachment and turn the internal divder into swiss cheese so the box is one big chamber.
you now have a large enclosure ready to be ported
For port sizes just ask one of the guys what the width and length the PVC tube needs to be based on the size of the box which will be around 2cft.