A Material worthwhile considering to produce a Sub Plinth or Sub Sub Plinth > Sub Plinth to mount the Speakers on is a Resin Impregnated Densified Wood Board, which has well known producers from Brands known as Panzerholz or Permali.
A more recent discovery has shown the material can be found with improved aesthetics as seen in the Link.
REZYWOOD - gefärbtes, stabilisiertes & hochverdichtetes Ahorn Holz | Designholz
There is quite a bit of info out there to assist with understanding why this material has been suggested.
Also as a user of the Material, It would be rude not to make known that very attractive End Sound is able to be produced with this material, when used as parts in a design for a support structure.