Video
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This is a really quick video showing some blanks used for casting epoxy as well as some handling techniques.
The handling techniques the creator notes is to use a mold release to make it easier to remove a cured resin from the mold. They also show the use of denatured alcohol to clean the mold.
The blanks they show look to be high quality and could be useful for a variety of small projects. It's probably much easier to just buy these then to try to make your own.