I seem to remember a thread somewhere, with 15" Tannoys using a sand filled sandwich.

The guy had scrap partition walling from a portacabin type of structure and used this, with the cavities filled with sand.

Huge and VERY heavy, but said it worked well.

I have built concrete speakers in the past that worked well, they were stolen/recovered a couple of times, the thieves just gave up and abandoned them, they were so heavy. I did investigate Lightweight concrete using Pumice stone and sourced a UK supplier for sacks of Pumice gravel at the time, but never progressed it.