I would say get a stealth box or a custom fabbed box, most people put it faced towards the rear of the trunk as far back in the trunk as they can, the stealth box mounts them aimed towards the sides and flush with the trunk but i dont know if they have a version for 12's or the fitment in a vert.
You bes bet is to buy some MDF board and make your own, its actually not too expensive.
2003 Redfire V6,
Show: Smoked everything, cobra front bumper, gt rear bumper, rear decklid and lower valence blackout with black mustang letters, 5% rear front 15%, black 98 Cobra rims front, 95 Cobra R rear black with chrome lip, black scoops and spoiler and exhaust tips
Go: CAI, True Duals with flowmaster super 44's, Bama Tune, full suspension (lowered, 8.8, 4.10s, UCA, LCA, springs, CC plates)