I found this on crutchfield, thought might as well (especially with the tweeters lol):
"If you choose to use a double-thickness of MDF for the front panel (this method is recommended — it provides an extremely strong, non-resonant mounting surface for the sub), fasten the two identical front pieces together using plenty of carpenter's glue and several sheet metal screws. Also, the double thickness will serve to strengthen the box as a whole.
(If you don't use a double thickness of MDF for the mounting surface, you should definitely plan to use bracing elsewhere in the box for added strength. In fact, it's never a bad idea to use bracing no matter what, especially if your box is larger than a cubic foot. The box will be subjected to extreme internal pressure, so the stronger it is, the better. The easiest way to add bracing is with 2"x2" strips of lumber. Glue and screw these along at least two of your box's internal seams before attaching the top and bottom.)"
