Hi Lynn, the magnets are square. One hint is with WayTags, any of the square tags can go on metal. The square tags can also go on plastic, glass, or anything else, but if you are tagging something that is metal, you will need to use a square tag. Your idea of attaching a WayTag to a piece of card stock or index card is a great idea! You can use a WayTag sticker – either square or round should work. For cans in particular, you can also use a WayClip with a rubber band. If you use the magnet, put it on the top of the can. If the can has a pull-tab, you can slide the WayTag right underneath the tab. Any of those methods work well. It’s purely a matter of personal preference which one you choose!