After an exhaustive search of practically every silicone pad/pan/trivet/sheet... available on amazon I settled on the "Heavy Duty Silicone Hot Pad Trivet, Circle, Large (9 1/8"), Dark Red":

https://www.amazon.com/gp/product/B00T92X2Z0/ (They seem to be out of the large now)

It is 6mm thick with raised ridges on both sides (included in the 6mm) and rated to over 200 degrees celsius.

Here is the trivet as it arrived:

I used a 220mm glass plate as a template:

Trimming the excess: