Shouldn't be too hard to fabricate something similar - if it did exist what's the betting it was asbestos.
So based on your description, my idea would be:
Two pieces of fiberglass infused cloth (the type that can withstand temperatures of 300c so that it won't go up in flames if sparks touch it).
Several magnets.
Suitable glue.
Glue magnets to one side of fabric 1.
Dollop glue on magnets and on random points of fabric 1 as well as all round the edges of the fabric.
Squash Fabric 2 down on top.
This glues the magnets between the fabric, and creates air pockets to act as sound deadening, as well as creating a one-piece sealed fabric mat.
Voila - one magnetised fabric sheet for grinding.
No idea if it would work though, I don't even own a grinder
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