I purchased solid color automotive graphics material and cut out my Mickey ears. It is specifically designed for it.
at least to us Dis Freaks.Laminate sheet on the front, then trim to size.I bought a pack of printable magnetic paper for $6.99 from Hobby Lobby (check out their wesbite for a discount coupon!) which you can print on with an inkjet printer. They are not waterproof but I'm working on a plan for that - probably using Modge Podge to seal it?
Bex