What is the perferred material for a TT shelf?

This will only set you back $40 or so give it a try; I use this on my Pro-Ject Xtension 10 and I'm happy with the results and it was easy and cheap.

IKEA Lamplig bamboo cutting board. The board has a grove on one side so I have that side facing down. $14.99 plus shipping.

Between the cutting board and my rack I use 4 RTOM Moongel Dampening Gels. I place the gel pads to the inside edge of the grove noted above; not in the grove but pass the grove. If you live near a Guitar Center or any drum shop they sell the Moongel there. Us drummers have been using this stuff for years. Package with six pads is $7.

This simple cheap tweak works great on all components as well. 

I have a DIY rack from three 3 1/2 in butcher block. The block is manufactured by patching together about 60-80 squares of maple with the grain running in different directions. My thinking, if you can call it that, is that the different orientations would preclude linearity in the wood thus reducing vibrations. I ran threaded brass rod capped with acorn nuts on each end. It sits on Herbies isolation footers. Seems to work great. 

I went with a 1.125" of solid cherry, Definite improvement over what I had.

Get a used Symposium Ultra on Audiogon for best sound.

Sand encased in a framed maple box with fixed bottom and floating top is my current method of vibration dampening experimentation. I made a few boxes of varying size settling on one with 6" total depth, subtract 1" for the piece of floating polished granite on which the TT sits upon leaving 5" of kiln dried sand, after allowing adequate time for some settling and an additional layer of sand. Naturally different TT and platform weights along with box depths/ capacities can vary results. Too much weight may over pack the sand causing it to be too uniform lessening its otherwise positive dampening properties. But I think I'm on the right tracks. I may have better results with a lighter piece of plywood or acrylic acting as the floater, and or a deeper sand box to play on.

As with most things having some variety to experiment and fine tune is never a bad Idea. Being accused a typical male as I'm sure many of us have at one point may of been what caused me to reach for the shiniest heaviest material I had in my arsenal knowing full well it may over do it. The lighter ply is still open for evaluation. I think I could live with current arrangement in comparison to some others below. But again I'm a guy so why would I not attempt fixing what isn't necessarily broken.   

I have tried others including steel target shelves, plys, blocks and air suspension made possible with the bike tubes and ingenuity. So far the sand case sounds pretty effective to me. Or the sand hasn't finished settling. After putting forth time and effort if results meet comparable to others the finished sandboxes look great so that brings with it a certain level of contentment. At least for a while.