One large sheet of microwave oven waveguide cover material 

Part number:

[MSC.MICAL]

Microwave oven waveguide cover repair sheet  200 mm by 150 mm by 0.3 mm thick.

Simply use your old waveguide cover as a template, mark the new one out with a pencil, and cut a new one.

If your microwave is arcing or sparking during cooking, it may be that the waveguide cover is damaged. This metallic type material, which is a blend of mica and is invisible to microwaves, it covers the aperture that the microwaves pass through to get into the oven cavity. When old damp or contaminated can react to microwaves by arcing.