Materials to reduce sidewall flex & vibration ?

Threaded bar with poly locks is a great idea.  

Baffle diffraction would be another concern if the widths or edges is altered. Or by using stone or composite tile, that sort of thing. Even slight changes can negatively effect how well the speakers image. 

Yeah, the edges of the added outside panels should be chamfered away from the baffle.

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.