Best material for an isolation platform?

I do much the same with my endpins for cello. One polarity of the shear wave travels on the solid brass outer surface while the other polarity of that same shear wave travels thru my chosen materials loaded in the center core. The rod is tipped off with a geometry that reduces blow back of reflections off the coupled surface. This blow back is the interfering energy that returns up thru the endpin pollutes the strings and bridge and becomes part of all the recurring notes..Just like in an audio platform or rack. Careful selection of materials and geometry are key.
Not 2 fast and definitely not 2 slow.
Tom

A geophysicist friend of mine has a patent on how to reduce interfering
energy that returns from the floor and back into the strings and bow of a cello. She has a method and science for select travertine stones
to rid the cello of its wolf note double peak that has harmonic coverage anywhere from 70hz to 18khz. The tail of the wolf note is parastic in nature and with its riddance the cello speaks faster plays easier and is more open and clear..and has greater acoustic output without the interfering energy attached to every harmonic.

A different remedy is at hand for those who realize isolation can only exist with the absence of matter. Creating greater impedance on or between 2 or more surfaces generates another set of issues as does spongy materials and springs between and under components and a shelf or floor.  Tom

Unless you have a reactively grounded rack that acts as a pass thru to the higher mass of the floor attached with points of a certain geometric shape that reject  reflected energy from the floor boundary. Tom