The finite elemente ceraballs mentioned by French Rooster look like they are worth trying out. However, would they work with a skeletal turntable like the Rega RP8 and RP10. I'm thinking there may be a slight risk of the turntable being knocked off the ceraballs by accident ?
Naim sell plastic rings to stick on the glass over the balls that provide the security against the glass sliding off. I only bought them for the top shelves.
I have a RP10, and the Naim isolation shelf works fine. Overall the eight isolation shelves have worked a treat. Until you experience the change it is hard to believe that inadequate racks can mess the SQ up so much. It is off course dependent on each setup, so some will have more of a problem than others.