I would think concrete could work....there are so many thickness you could work with. Thicker for the woofer. There is light weight concrete and you could texture the inside with ie. polystyrene or some media to make it more textured. ...the problem is how the hell are you going to test them....to much work in my view.
Concrete Audio Speakers...Speaker Enclosures made out of Concrete ?..
Could concrete speaker enclosures give us the most realistic sound ?
https://www.concrete-audio.com//en/#lautsprecher
Not connected with this company in any way
https://www.concrete-audio.com//en/#lautsprecher
Not connected with this company in any way