Hey!
When I replaced the transformer in my pre, I tried faucet rubber washers and 1/4" cork cut to fit from cork sheet. I listened to each one for a while and at the end I preferred the cork board. Not sure why but cork sounds better to me and seem quieter. Care must taken not to tighten the nuts and bolts too much though.
By the way, I used to use 1/4" sheet on top of my old Rega Planar 2's glass platter instead of the supplied felt mat, liked the cork better.