The Subs are actually screwed to the plywood and then the driver access and output ports were routed out of the plywood to match. The wall to the rear is double 5/8s drywall with Green Glue as it was going to be the front wall before I bought the subs. Three windows were filled in with rebar and concrete as well. The entire cavity is filled with the pink stuff to the ceiling. I made the grills out of matching Klipsch cloth.