@lewm That's why I should open a new thread! There seem to be two broad approaches to 301 plinth design. One is an open-air skeleton to let the noise out, the other is to fill the voids as much as possible. I am experimenting with the second approach.
The SME mounting board is 18-mm chipboard, and I am infilling with 27-mm Medium Density Fibreboard (MDF) because I have it lying around. MDF is used in many speaker cabinets because it is an excellent absorber of sound. Garrard originally supplied springs and mountings to suspend whatever mounting board one chose. They also supplied four rubber washers to slightly de-couple the chassis from the mounting board.
My approach differs from others I have read about. I don't have to consider the look of the internal plinth because it will be hidden inside the SME one. The original SME dust cover will stay in place to mitigate airborne vibrations. I can experiment by removing the Garrard-supplied suspension components, and the rubber washers. The mounting board and every layer will simply lift out, so if need be I can experiment with different materials, including Baltic birch and the huge range of Australian hardwoods, such as red ironbark, and my favourite black-heart sassafras. I would like to experiment with Corian for the mounting board! I think Corian and engineered stone has just been banned in Australia because of silicosis! Mind you there is a lot of silica in sassafras too.