These Tetris pieces have a round hole in the back to take a 10mmx1mm cylander magnet or Jewlers pin in the back. The hole is just the right size that magnet will press-fit and stay in place but I recommend putting a bit of thin cyanoacrylate glue on top for a more permanent hold. If you don't have a 3D printer I sell these on Etsy here: https://www.etsy.com/listing/150561288/magnetic-tetris-piece-note-this-listing?ref=shop_home_active