How about using a dampening mass that is used in vehicles to dampen the vibration of the doors and other platings etc, for example http://vibratec.se/products/special-...s/noisekiller/

I have really good experience of this kind of solution. http://www.caraudiodirect.co.uk/chp-...ste-1kg-single

Lautasella vaimennusmassa.jpg

The needed thickness is of course much more than the recommended but you could always do it in thin (3mm) layers to get the required amount to fill the platter.