How about flattened bottle caps? You can punch a hole with a crop-o-dile or awl in the top edge and put a coilless safety pin thru. I made these as magnets with a 1" circle punch and some retired SU paper. I filled the top with UTEE, but you can buy 1" epoxy domes that are self adhesive and less dangerous than molten plastic resin (ask me how I know about danger
) Make sure you buy bottle caps that have the plastic inner liner already removed - can be a big pain. I think you can even get them pre-flattened and punched, but I just flattened them in my big shot with a No More Shims mat.