I've often wondered about the practicality of using of light-weight concrete as part of a CNC machine. Foam concrete is used in the building industry but is not easy to make in your own back yard. One option I considered is to use small polystyrene beads (as sold to make those incredibly impractical bean-bag chairs) as all or part of the aggregate in ordinary concrete. We're not looking for anything like the strength required to build a road bridge and this stuff might well provide effective vibration deadening with a high bean content at a fraction of the weight.
As with many of my best ideas, I have had the bag of beans tucked into the roof of my shed for years but have never actually got round to using them.

Kit