Maple platform under turntable, How thick?

I have used a 3" one from Michigan Maple Block; good and not expensive. I have used Star Sound brass cones and FIM ball bearing saucers. [Dealer Disclamer]. FIM platforms show up on used on occasion and are usually good deals. These both work but do not sound the same. There are many on the market and most are effective to one degree or another, I doubt if anyone has tried them all. If anyone has they have WAAAAY too much time on their hands.

One thing to keep in mind is whether you are trying to couple or decouple the platform from the floor. Cones usually couple, bearings decouple. One thing not mentioned is using partly inflated small tire tube to isolate the platform. I use to use slate, which is better IMHO than granite but harder to find. I used the deflated racquetball method under it, I haven't heard about this trick for a long time.

As I said, there are many out there. If your floor vibrates then you want to isolate your table from it; if not coupling is fine. The only general rule I have discovered in audio is that there are no general rules and that what works best is determined by the particular circumstances of your system. That is why after 50 years in it I still haven't achieved Audio Guru-hood, I don't try to give absolute prescriptions from a distance; don't trust those who do.