A strong box is essential in a high output system. Your box should be free from air leaks and braced internally if the volume exceeds 2 cuft. Internal cross bracing is far more effective in making a rigid box than doubling the wood thickness. Doubling the wood thickness adds a great deal of weight to your vehicle, and the added wall stiffness is less than stiffness added by cross bracing. Try to cross the braces at the same point so that they add strength to each other.

The inside seams of the enclosure can be a source of air leaks and potential weakness if the joints are not cut perfectly. One way to strengthen these joints is to use gluing cleats on the inside seams.