On 26 Jan, 15:52, "Graham." wrote:

Large sheets of Mica are available from suppliers of microwave oven spares

That's not mica though, it's some ersatz synthetic "mica" and not
transparent for window use. Easy to cut without breaking though, as
it's much softer in bulk.

http://mutr.co.uk have the real stuff, but even their's isn't big
enough for windows.
http://www.mutr.co.uk/product_info.p...67cc51895b8cdd