you could try transistor insulating mica..
The thicker types can be split with a boxcutterknife.

"Chuck Harris" schreef in bericht
...
Mica is harmless, but the mining operations are not. Mica is almost
always found interspersed with asbestos.

-Chuck

Tio Pedro wrote:
Try your local hardware store. Ask for a replacement
"mica window" for a wood stove. Peel and cut to
thickness. EPA does not regulate mica, it is harmless.

Pete