Materials to reduce sidewall flex & vibration ?


I am wishing to reduce sidewall vibration without adding reinforcement 
within the cabinet. I'm curious about fiberglass and other products that can attach/bond 
permanently to the sides yielding the highest stiffness to weight ratio.
ptss
Hello rotarius. I appreciate your suggestions are very relevant so likely you
have some knowledge. Do you have some more specific suggestions? Thanks.

It's hard to be specific because induced vibration and resonances are so dependent on the masses coupled together and the natural frequencies of all the materials of a given system (speaker).  At work, I have access to vibration analyzers, so if I had to attempt an experiment like this, I would use some equipment to quantify the results.  Like someone suggested earlier, you could use materials that dissipate vibrations in the form of heat on the inside walls without much thought.  Just because something you stick on the panel is very rigid, it does not guarantee anything.  I mean it will still transmit vibration and it will resonate at some higher frequency. Is that better for your sound? 
Thanks rotarius. I very much respect the benefits of measurement with equipment; but it's not available to me.. You mention " you could use materials that dissipate vibrations in the form of heat on the inside walls without much thought ". If so, why could the same materials not be used on the outside? I'm simply looking for the best material for this. So far it seems like an epoxy with fillers of fiberglass,carbon fiber and lead slivers to both provide rigidity and dissipation of vibrations by turning them into heat. Your thoughts?