I'd use a thick piece of butcher block maple. It sounds better than foam. I have even read some accounts of where people have replaced their VPI plinths with solid maple to good effect. Just my opinion, but soft absorptive materials generally tend to deaden the sound of a TT.
Home made turntable isolation
After researching many different issolation platforms I have decided to make my own to hold my TNT JR, based soly on price. I am going to have two peices of 10ga stainless lazor cut to my spec. (I call on a couple of accounts w/ lazors. The problem is the core, I have researched and found fiberglass suppliers carry many different materials high and low dencity foam. The one I would use is a 1 1/2 inch honeycomb but the honeycomb stands on end and I dont think this would be the best option to dampen vibration. If it ran the other way it would be my choice hands down. Also balsa wood is availible. I will have about $125 into something that would be $900 (I checked)What do you guys think on the core. David