Re:Playing with straight pins
Another possibility is to cut up a magnetic sheet:
Click at the bottom of the page and then click again to get to the third "magnets" page. $6.75 for a 12"x12" sheet 1/32" thick with adhesive on the back.
If you ever need to flip the counters then you would need some kind of metal to put in them, not magnets. Sorry I can't find a URL for that.