This popped up in FB a few days ago. Price seems reasonable, but of course I want to see complete NRC tests. They say that the product will be sold in Lowes and Home Depot!
I remember as a kid going to visit my aunt and uncle - their house had wool carpeting throughout. That house always smelled amazing. The first thing we did was take our shoes off - wool felt amazing as well. There really is no synthetic substitute for wool. Nothing beats a good pair of wool socks, summer or winter, or a Pendleton shirt.
Something like this would reduce the reflections inside the cabinet. Max effectiveness probably starting around lower midrange.
IMO there isn’t much you can add to the panel to reduce it’s resonance. Since the panel should already be pretty stiff, a big mass would be needed to have much influence. Better bracing would likely have a much more effective result.