"half inflated" kids scooter innertube
I used something like that in the '70s when making holograms, with equipment mounted to granite plate. Worked great for isolation, I'm sure it would work for audio use.
Simple and cheap!
Isolation for Granite Surface Plate
I see from other threads that using a granite slab as a surface plate for a turntable is problematic. I am currently using a .75 inch thick slab as the top of a Zoethecus rack. It is isolated from the rack by 4 Hudson 1.25" diameter silicone 20 duro bumpers. Can I do better with a different isolation feet? My wife is very happy with the way this setup looks, so I'd like not to replace the top shelf with a different material if I can avoid it.