Shielding from microwaves

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).
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