Have you considered commercially available damping materials used (for example) to isolate electric motors from their mounting surfaces in boats to eliminate noise?
I'm not an analog guy, so I really don't know if you want to isolate the TT from the rack in the same way, or if you just want a steady base for it- from TWL's comment I would suspect you may not want to isolate it per se because a hunk of maple really wouldn't be as effective as a couple of rubber pucks of the right consistency, but the maple reportedly sounds better. But, if you do want to isolate it you should be able to buy a sheet of vibration damping material engineered for any of a number of given weights / vibration frequencies at a pretty low cost to sandwich between your sheets of steel, which would make a nice isolation platform. If you could pin down what characteristics you really want, there will be someone with an isolator material that fits your needs.
check this out: http://www.vmc-kdc.com/pdf/pad_e%2B.pdf as an example of what's out there