Materials to reduce sidewall flex & vibration ?

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.

Please help me understand this. Is this a work around for a poor cabinet design-build? I can't imagine having to do something like this to my speakers.

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.