Round Magnets (you can purchase a 6 pack of magnets at staples for about $3.00)
Generously fill the inside of the bottle cap with hot glue.
While the glue is still hot place the magnet into the glue. It doesn't matter which side because both sides are magnetic and will stick to your fridge.
Let the glue cool for a minute and then place on your fridge, or your locker!