Maple platform under turntable, How thick?

Best you not entertain the half measures being suggested. The serious audiophile would go directly to an electron microscope table. These can be had for about $100,000. They will tell all that you are serious. Many years ago I visited someone who had such a table. He was so proud of it. I could not tell it made an difference. He was shocked.

I was happy with my a la Mapleshade 4" maple block on cork/rubber pads, untill I decided to try (based on A-gon thread) aluminum top plate on sandbox. Bunch of transistor heat radiators are bolted to the bottom of 3/4" anodized aluminum plate and submerged into 30 kg box of sand - all unwanted vibration are completely sunken through the huge contact surface and died out naturally in the sand . The whole thing is supported by 6 big and thick sorbotane pucks - look at the sorbotane site how to calculate the optimal geometry of the sorbotane pads to minimize natural frequency of the whole assembly.
I got much jucier sound combined with ink-black background comparing with the maple platform - I guess I'm done with isolation platform.

I had vibration problems with my turntable on my raised wood floor. I had to sneak up on my turntable to keep it from skipping. I replaced my old rack with a Steve Blinn rack here on Audiogon with 3 inch maple blocks. Now I can walk up on the turntable without any problems. The sound from my whole system is much better from the new rack system. I would recommend the 3 inch maple blocks.

As thick as a tombstone.

Whatever thickness you can get, I would suggest buying a bunch of squash balls and putting them underneath. It's kind of like a cheap man's Ginko cloud, but with a maple flavor to it. You'll probably need at least 8-9 balls for the weight.

One of the better things I did for my TT.

Enjoy,
Bob