Hi, I am having a little microwave experiment, and I want to shield some
cracks from leaking microwaves. A friend 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).

A square or disk of mica is used in the cooking cavity of most modern
microwaves to cover the waveguide aperture. If you remove it, you can
see the cylindrical antenna on the magnetron. The mica window is
there simply to prevent food debris from entering the waveguide.
It is my understanding that the mica is pretty much transparent
to the radiation, I wasn't aware of any significant diffraction.

My first Plustron microwave which would have been well over 20
years old by now had pieces of ferrite material embedded all around
the door frame, presumably to choke off any residual radiation.

Your microwave oven is a Faraday cage, why are you worried
about its safety?