Materials to reduce sidewall flex & vibration ?

creating a composite panel speaker

The answer is obvious. Smart metal. 

cascade spray

Thanks johnk but the damping spray you mentioned,although it does have its application benefits, will not contribute to my goal of maximizing rigidity..  "I'm curious about fiberglass and other products that can attach/bond permanently to the sides yielding the highest stiffness to weight ratio." So far my research seems to indicate that epoxy filled with fiberglass or carbon fiber,or carbon nanotubes, combined with high mass material is the best option. I continue to research and appreciate contributions.   

Thanks Geoffkait. Yes smart metals have fascinating potential,along with gels that instantly harden upon attempt at deformation, but I feel the momentary time delay renders both non ideal. 
I think constrained damping can be more immediate. Of course I'm just thinking; I'm not a materials scientist---aha--you've given me the idea to search for the 'chat rooms' of materials scientists discussing cutting edge damping materials and techniques. Appreciated.