My DIY JBL-4350 has two 15" woofers, each in a 5 cu. ft. enclosure, for their enclosure I used two layers, 3/4" birch plywood on the outside glued to 3/4" particle board on the inside, that is 1 1/2" total thickness, it is heavy but as solid and resonant free that is practically possible, this enclosure does not require any internal bracing.
