[WGH]

Blu-tac or silicone are great ideas. Sealing/varnishing both sides of the panel with exactly the same number of coats should minimize any cupping.

A simple science experiment you can do at home with scrap lumber is to find a 1x8 pine board and wipe one side with a wet rag and watch what happens. Humidity works exactly the same but slower. Solid wood table tops are always finished the same top and bottom for this reason.

[JakeJ]

I have a related question, if I may.

Since the problem with expansion and contraction exists with all hardwoods and some other materials, would panels made of stone such as marble or granite work?  Making the stone/MDF interface as intimate as possible should offer excellent damping as well.

[WGH]

... would panels made of stone such as marble or granite work?

Yes. I have had excellent results using Richlite, a very dense paper/polymer laminate that can be cut and shaped using woodworking tools. I got the idea from Albert Von Schweikert, his speakers use stone on the inside.

"The Audibility Of Cabinet Panel Resonances and Pat. Pend. Method Of Reduction Of Audible Coloration"
by Albert Von Schweikert, Chief Design Engineer, VSA Corp.
Copyright July 2009 by A.V.S.
https://www.audiocircle.com/index.php?topic=70291.0


I have a thread about how to use Richlite

Von Schweikert VR2 Modifications
https://www.audiocircle.com/index.php?topic=72808.0


My current speakers, the Salk HT2-TL, have 1-1/8" thick walls with a 1-3/4" thick solid mesquite front baffle so I didn't need to apply Richlite, the cabinets are already pretty inert.