What is the perferred material for a TT shelf?

If your table is a non-suspended design: Two shelves of Baltic Birch Plywood separated by a slightly inflated bicycle inner tube, with the table sitting on three roller bearings arranged in an equilateral triangle. The most effective isolation short of a Minus K or similar platform (which cost in excess of $2000).

I purchased a Jean Nantais Lenco Reference Mk III a year ago.  It's a suspension-less design.  Jean recommended a 1/2" slap of marble laminated to a 3/8" think slab of acrylic that is the top layer.  I put Herbie footers under this platform to isolate between my heavy Merrill Stable Table stand.  Works great to my ears. I used maple remnants (beautiful ivory color, all polished and finished off) purchased from a local floor and tile shop - $200, and the acrylic from my local picture framing store - $150.  Cost effective, relatively speaking.

My TT and sits on a 3/8" thick piece of granite.
The granite sits on a 3/4" thick MDF shelf.
Between them is a 1/10" layer of foam
- i.e. the type with holes that you put under a rug to stop it slipping.
- this kills all vibrations in both the MDF and granite
Between the MDF shelf and the steel support for the shelf are 1/10" thick pieces of sorbothane - this kills any vibration from the rack & floor

The TT itself weighs around 25 lbs and has 3 ball bearing feet that sits directly on the granite and between the ball bearing and the TT plinth is another piece of sorbothane

I’ve tried lots of other methods and this is the most vibration free I’ve managed to get it to date.

I use the granite/foam/MDF for all my opponents - I just adjust the size of the granite/foam to a size closer to the size of the component - e.g. my phono stage and DAC sits on a 5/16" thick, 12" x 12" granite tile  which sits on the MDF with foam in between

From this experience I believe the key is to build a shelf from a sandwich of vastly different materials

But - I'm always on the lookout for new ideas :-)

It's always good to hear DIY solutions...

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.