What is your favorite material for loudspeakers?

Just looked at link! Yep, as I remember......
Need a new thread:: 'rocky-bullwinkle trivia'

Guidocorona
I suspect you could build a speaker enclosure to operate like a fine string instrument.
You could make one, or perhaps even a matching pair.
There is almost no possible way, for a reasonable amount of money, to mass produce such an instrument.

As for a practical (advisory) perfect enclosure, why not concrete? Heavy and inert, it would add nothing to the sound of the drivers. Add dampening to taste.

I'll stick to my panels.

Concrete blocks? Were they sand filled?
I was speaking to a system I saw once where the speakers were poured in place. I suspect the chimney block effort was sabotaged by the front/rear panels.
ALL concrete is the way to go! A couple inches of Portland will never rattle.

From MT Flatten?

Upsidasium was the metal (mythical) discovered on Mt. Flatten.
Don't you remember Rocky and Bullwinkle?
Captain Wrongway.......but I'll save that for later!