Once the box is squared, glued, tacked and clamped, I affix a pre-cut piece of 3/16" pegboard to the top using glue and shorter brad nails. The pegboard helps to stiffen the frame and tied the piece together. It also allows low frequencies to pass through the panel, hit the wall and return for even more low-frequency dampening. For this reason, it's important to suspend these panels 3-4 inches away from the wall or ceiling!