Materials to reduce sidewall flex & vibration ?

No Rez, designed by Danny Richie of GR Research. It is expressly for absorbing speaker enclosure wall resonance, being installed on the interior of the enclosure's panels. Very effective.

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. 

You could certainly go with granite, finding a really good installer to cut and glue the sides and top. Corian could certainly be tastefully done as well.a bit easier to do a cutout in the front for the speakers as well. If it works really well you could start a business.

Hard floor tiles! The thicker, heavier type without self-stick bottoms. They add mass, which lowers the cabinets resonant frequencies, and damp that resonance.

Texan Sonic dampers for speaker cabinets and subwoofers are, alas, no longer available. The problem with trying to use materials to damp the cabinets resonance is that almost all if not all materials trap as much as absorb. That's why Tekna Sonic dampers were so effective, they were a mechanical diode. They were also excellent for use on room walls as it tuns out. I used to use Tekna Sonics for my isolation stands way back when was big customer. IIRC I bought the last Tekna Sonics available from Music Direct eons ago.