Hi, I am having a little microwave experiment, and I want to shield some
cracks from leaking microwaves. A pal said to me that a paper-type
called "mica" is used for such reasons in microwave ovens. Actually what
he told me is that it allows microwaves to pass through, but it blurs
them so as they are not focused as before.

Is it true, or does anyone know any better way to make leaking
microwaves less harmful (apart from Faraday cage, this is too hard for
me to make, I suppose).