I made my set of magnetics about 10 years ago from chrome reflective magnetic sheets that I ordered online somewhere... sorry don't remember where but surely you can find them somewhere to order from. They were originally used on my black MR2 but I've carried them over to the BRZ where they still provide a very clear visible number on the blue. The sheets were pretty thick about 2mm thick, and I have never had a problem with them coming off at autox or on track. My tip is to use extra bold or puffy letters to maximize the magnetic area. To be honest I don't really sweat it when I throw them on if they or the area I put them on are dirty... i guess because race car.
