You need a router bro. Other than that , some 45s in the corners, route the port and some sort of bracing top to bottom preferred. Enjoy that sub, the new ones are not even close to their level !
Box I just built for 2 C3 15s in a Pontiac vibe.
I love mine. They have small trunks as well so you have to looking for a large single sub set up or a couple of small subs if your going to puts audio in there !
I tried full power.......blew up alot of **** ! All 16 subs went ......4 of the coils came unraveled and then the rest snapped speaker leads from too much excursion.