I cut one square of black posterboard and two strips of white posterboard. You could use any brad, but I happened to have tiny ones for scrapbooking, so I used those to anchor the white strips to the black background. (I just poked a tiny hole all the way through the two layers of paper and then shoved the brad through.) I then put a piece of packing tape on the back of the brads so they wouldn't scratch my whiteboard. I grabbed an advertising magnet off my fridge (you know,...the kind you get in the mail), cut it in half, and glued one strip to each side of the back of the black posterboard. And voila...a greater than/less than/equal to tool to use on my whiteboard.
You could use it with any magnet...including regular magnetic math manipulatives. The above photo shows a problem before it's solved, using base ten pieces. Then all the child needs to do is pivot the white strips to show greater than, less than, or equal to.