Materials to reduce sidewall flex & vibration ?

Thanks bdp24; however, in this case I'm only interested with
material that can be bonded to the exterior of the speaker. I'm hoping 
some have experience in materials such as fiberglass and carbon fiber
as well as with granite,etc. 

donjr, that's correct; and I would say at least 95% of cabinet sidewalls flex.
I don't really know if it will improve sound any; but it's idea I want to investigate.

Thanks guys.
Dentdog,Granite sounds ideal. For a business all I need is a warehouse and laser cutter for the granite; but, I'm retired so that won't work.
bdp,hard floor tiles,perhaps black, are interesting.
I mentioned fiberglass/carbon fiber as they are more rigid that rock by weight and I'm hoping someone here has experience with them, perhaps automotive or art related.

But, panels 'inside' the enclosure change the internal volume-
an area carefully calculated to work with the drivers used - to provide the right 'damping' for the drivers. This is why I am focused on the exterior.

FYI. "Epoxy granite material has an internal damping factor up to ten times better than cast iron, up to three times better than natural granite, and up to thirty times better than steel fabricated structure." Answer found in other forums.  

Thanks to bdp24 and meerzistar for contributing to reducing side wall flex and vibration.

I'm still researching;here's new info..
"  Hardinge as well as many of these; others are quick to point out that the damping properties of epoxy granite are about 8x those of cast iron."  I'll be looking to confirm this as it appears very relevant. 

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.

Thanks soundsrealaudio for your direction. I agree with you and am wondering what materials would do this best.

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.   

Just to clarify soundsrealaudio, I am seeking a composite of the 'most rigid' and highest damping' materials to use. So far the filled epoxy appears the most efficient. 

Soundsrealaudio, I agree that a composite material is ideal.
I'm searching for the 'ideal' practical solid,high mass deadening
sheet and the ideal adhesive. I want to create maximum stiffness
with highest possible damping. It seems carbon filled lead epoxy
is a contender. I'm hoping someone with industrial experience may
have something to contribute. Shipbuilding, aeronautical, train building
or massive motor vibration reducing techniques perhaps. 

Hello rotarius. I appreciate your suggestions are very relevant so likely you
have some knowledge. Do you have some more specific suggestions? Thanks.

Spoken like a dealer.

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?

Thanks rotarius. As a fellow tinkerer I'll let you know that I do consider the final
look to be important and have decided to use good quality leather,in black (speaker color),as necessary to refine the look. I've already got the leather. I've found 3M Super 77 spray glue to be ideal for leather;just the right tack time then a nice permanent bond.