Materials to reduce sidewall flex & vibration ?

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.

You know I've thought about this off and on in the past and wanted to come up with a cheap DIY that would test if it would be worth pursuing.  I came up with one idea.  If you can imagine a bag of sand (or better lead shot) say 12" long 3" wide and about 1" thick - now sew as many of these together (kinda like a bullet belt) as required to drape over your speaker's top and sides.  Now the back wouldn't be covered unless you made a belt for it, but this method was only just a test I could easily fabricate and try out and would only be applied during listening sessions.  It's not a very aesthetic thing but it could work I think.

If appearance is not a concern, a C clamp attached to each pair of opposing walls. One top-to-bottom, one left-to-right on the upper half and another on the bottom half, and one front-to-back, between drivers. Kind of drastic, but it would certainly reinforce the enclosure and prevent flexing of the enclosure’s panels, It would also simultaneously raise the resonant frequency of each panel and the enclosure as a whole.

I got few area rugs from flea market near-new and same and hung around speakers on three sides and paid only $50 for all three.

Heavy paving stones have long been placed on top of subwoofer cabinets by owners, as have bags of sand or lead shot.