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 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.