Michael's (art supply store) sells rolls of magnetic sticky sheet that are about 9" wide and 24" long. cut in half it makes two sheets, each about the size of a sheet of paper. print your numbers from your PC, peel the backing on the magnets, stick your numbers on and you're done.
I broke down and bought numbers from a sign shop - magnetic ones. It was maybe $25 for the whole deal? I put yellow duct tape to keep them from peeling off. See photo.