The Great Stuf could be trusted to push out the panels and maybe loosen the fasteners. And w/o you'd have 2 flapping panels/side. Taking a bit from Bob Brines and another from Art Ludwig I'm about to try a consrained layer with 3/4" Mdf then 30# roofing felt then 1/2" Hardi-Backer. Held together with polyurethane construction adhesive. If I ever get done with the freakin' radio! Watcha gonna use fer woofers?