"Bracing
The goal of cabinet bracing is to make the cabinet as rigid and resonance-free as possible. Bracing made of 2x4’s or
75mm (3-inch) pieces of the (birch) ply should be applied inside the cabinet. The braces should be placed at slightly
odd intervals on the panels and liberally glued and screwed down on edge. The glue on the braces accomplishes all
the stiffening needed so be careful to apply enough glue. Use one piece of bracing on panels between 300 mm and
600 mm (12" to 24"), two pieces on panels between 500 mm and 1200 mm (18" to 36"), and three pieces on larger
panels. The baffle cutout disc that is normally discarded can be glued and screwed down to one of the box panels
for additional stiffening." - JBL Professional Encloseure Guide

Worked really well on some enclosures I built for Spkrman.