Quoted from jawjaw:
What mylar are you guys using for the magnets? I have some that are a whole circle that will cover the entire magnet. Is it a bad idea to use that?
You don't want the magnet covered. You need to cut out the area of the magnet. Some have suggested the pre cut ones for iron man work well for the Sparky magnet. If you have an uncut sheet you can probably find a socket or something that is the size of the magnet and cut it yourself.