I have quite a history of experiencing footers under Plinths and Sub Plinths and equipment.
I used Manmade Products such as, Composites both Hard and Soft, Putty Type Substances, Sand Box, Sorbothane, Sorbo-Rubber, Foams of Different Densities (This always available), Adjustable Footers from certain brands with slight differences to the designs, AT 616 Pnuematic Footers, Solid Tech Feet of Silence Footers and Metal Spikes.
For Natural Materials used, I have Cork ( This is always available) and Black African Hardwood such as mpingo.
From the list of the above, I can assure that the mpingo was the shortest used on a trial and not kept. The Composites and Putty Types were superseded in use in the early days and the Sand Box and Sorbo Type Materials were used, the Sorbo Rubber for very heavy equipment.
To Date I still use foams of various densities and Cork, these materials are very good at tweaking a colouration out when detected, when working with these materials, even mixing them. By adding them in layers to increase the depth, the process of building up the layering can significantly contribute to gathering a loose bass and improve on this. The real attraction is that it is cheap to have available in a 5-8mm thick material.
I went onto the 'AT 616 Pneumatic' which was a very impressive introduction and that was bettered by the 'Feet of Silence', when directly under the Source. I use these to date with the 'FOS' supporting the source and the '616' Supporting the Sub Plinth.
My Local HiFi Group loaned the '616' and 'FOS' and become converts.
They then brought in readily available feet to compare for the ease of purchasing and for the fairer pricing, the footers the Group have bought into are Iso Acoustic, of which I know some use the Gaia and some others use the Orea. As these are offered at a discount through a source a Group Member knows, there are systems utilising multiple sets of Iso Acoustic Feet.
My last demonstration of an Idler Drive in use was a Garrard 401 > Origin Live Illustrious> Sumiko Pearwood which was mounted in a Bamboo Plinth and a Sub Plinth of a material l supplied with AT 616 Footers. To this date from all recollections, the 401 on this occasion was the best I can recollect one performing during a demonstration or from personal usage experience.