You might try mixing Styrofoam balls or vermiculite with your concrete. The right blend will maintain the density quite well and the drop in weight can be dramatic.
Zaph SR71 kit with concrete baffle
I've been working on this on and off for a couple of years. I had to learn how to use a router better, how to cut holes with a jig, a bit about concrete, and after a bunch of tries I've got the front just about finished. I'm mounting the drivers inside oak rings because I didn't think it was a good idea to try to attach them directly to the concrete. I'm gonna use some kind of adhesive to hold the rings in place, and then also glue the baffle onto the box. The initial plan was to then put ceramic floor tiles on the rest of the box to make what will be about the most dense, unresonant box possible I think.
https://imgur.com/fzHUooF
https://imgur.com/fzHUooF