If you some handywork skills and basic tools, building a fiberglass enclosure yourself is "relatively" easy albeit a little bit time consuming. Blue painters tape the rear area where youd like the sub (right pocket area in particular). Cover completely with plenty of buffer tape. Lay down some fiberglass MAT not cloth from home depot, dab with fiberglass resin. Lather, rinse repeat, until you have a few layers covering the desired area. Let it cure overnight. LEAVE CAR OUTSIDE! Resin smell will permeate garage and house and significant other will likely threaten to murder you if you dont. Remove shell. Add additional layers until content with thickness. Buy or cut 10" MDF speaker ring and glue to thick dowels and glue into position in shell. Resin pegs in for further support. Drape enclosure in felt from fabric store and glue taught to the shell. Paint resin onto the felt and let harden. Trim excess fabric from shell and lay fiberglass on hardened felt until desired thickness. Once all is cured, sand, bodyfill if needed/desired, sand some more and cut out resined cloth speaker ring inner and cut a hole for the wiring post block. Then just glue down speaker carpet to your new enclosure.
I know it sounds involved but honestly its pretty easy and WAAAAAYYY cheaper than any premade mass built phantom box you'd likely find out there.
