Nude Turntable Project

Corian might be ideal. What about the material used by Kenwood which people call faux marble. It's ARCB - Anti-Resonance Compression Base. It makes up the plinth of the KD500, 600, and is used in the L-07D.
Contrary to popular belief, it's a mixture of ground limestone and polyester resin. I imagine you could add liquid acrylic pigment for the color of choice.

I think this could be sculpted like clay or put in a mold. You might find the best solution is a combination of materials - resonances tend to cancel.

Henry,
I suspect the coupling - 3 rubber grommets are the root of this evil. It won't matter how much mass or how stable the base is, if the coupling is inadequate.

Can't say I have a solution, but I'd try something like a thin bead of silicon caulk, or possibly the sealer they use to couple a speaker driver to a cabinet. I wonder what Mosin uses to couple Saskia to the slate plinth.

Regards,

Now that I see your last post, what you're describing is a low frequency oscillation (gets louder). If the stylus is stationary, does this originate in the table or preamp?