Granite Slabs

Ebay or FB Market Place will help source Granite using a local search. 

Monumental Masons use thicker slabs than Kitchen C'tops, the M' Mason will have off cuts that can be tidied up.

 I have wrote on this forum teirs under a Speaker bring the most attractive results.

Three Tiers of Separators and Two Platform Tiers is a excellent starting place and able to be achieved in the low hundreds if purchases are prudent.

Granite at 50mm - 75mn thickness is fine as one of the platform tiers. Granite near 20mm thickness does not deliver as the thicker slab does. 

gndrbob

regardjng your comment on Amazon selling “machine plates”, I’m a bit new to this area, can you provide a bit more color on what I should search for on Amazon?  When this rookie enters on Amazon search, it just shows workout platforms.

greatly appreciate the help

https://www.amazon.co.uk/HHIP-4401-1597-Granite-Surface-Plate/dp/B08BXMX46W/ref=sr_1_2?asc_source=01H8HFYCRM99TJ9FED7FSB1ZXC&crid=18KGO8XAFZZ7F&dib=eyJ2IjoiMSJ9.232CKKt0hr-jungdZugkjZ7qNo185dO8bd0NCmJGuEcGZHhLVVtqGBZU1r-B34P8gn0Gjbv_CBNTREyJ5hzmASRzXidIC308gb8LCXmRle2C5SCbcMrap21h9thdmFTF8SUVgQtPZF0LSET-yCL6PJnBkedAq4M4dLnJ3KKNHY-qej5FAUjkjBHDgDzpCuXcBkOkjUJN_3hrAIz7oEhwfzHjcykLIr9he9yZJ9q8_o14oarB567arAL2P8rWrYZQbnbJfh-pVAqNTNW6JLZEMRIb-WS5v1jnmQWVPcmyTq0.VxXdDrpTsovlgD5A-YhqYzXRSNm62KG5W9IE_dCj4NU&dib_tag=se&keywords=engineers%2Bsurface%2Bplate&qid=1764097976&sprefix=Engineers%2BPlate%2Caps%2C387&sr=8-2&tag=snxgb19-21&ufe=app_do%3Aamzn1.fos.95fd378e-6299-4723-b1f1-3952ffba15af&th=1

https://www.ebay.co.uk/itm/365966181421?_skw=Granite+Surface+Plate&itmmeta=01KAY7990GBRKVA4NR48D31503&hash=item553548c42d:g:-qQAAeSwqUVonOL6&itmprp=enc%3AAQAKAAAA8FkggFvd1GGDu0w3yXCmi1eJQZCaXj8UaQUEpHeLXThzTsDOeyKnOfwR5RgLI19LQdiAeKX4rclm1IjzfHFROUpqE83xDEjA4wzAzQnHaHIZyDenV2dY1iQO%2B9M7810k1%2Fr2EZ29rph1qArLOWMloV33FUW3jNmae73FIrkd6CNBXHbkIh8i6hgEQJng2rwjkHnndhXvY3osz7uhOtrRoITWoyuygyVNBNpCmknQNYk%2B5pOJAvn7zCKfR2H3R6Z3VBgL9qPQHRPBVc8dtihvW4y%2BZNDWsUqcUOqK4lV4DcmgmSWep4UknPVgdD6Rn9pX4g%3D%3D%7Ctkp%3ABk9SR8SQpcfXZg

Thicker is better, common sense of course should be applied. 
if you will be ordering from granite cutting shop, don’t tell them that you’ll use it for sound system. Tell them you’ll use them as kitchen cutting boards so it won’t cost you a fortune. This strategy actually helped me to save a hundred. 

When we moved house around 10 years ago, I had solid granite kitchen counters custom cut. I had to buy either a sheet of granite (which is quite large) or a half sheet, so I bought a full sheet and had them cut and polish two smaller slabs to my specification to use as bases under two of my turntables.  It worked out very well.

Suggest that you contact a granite business - they will always have part sheets left over from jobs and should be able to supply cut down pieces to your specification.  Be aware that they will be fairly heavy and need a good solid base underneath them - it can even be as simple as a couple of standard 8x8x16 concrete blocks turned on end or it can be something more aesthetically pleasing.