The best magnet for bottle cap display: round ceramic magnet with a diameter of 1 inch and a quarter inch thick. Fills the entire inside of the cap for a substantial feel. Not available at your local hobby store: these are bigger and thicker!
Turns any bottle cap (raw or used) into a great fridge decoration or collectible, with no need for glue, since crown caps are steel and thus magnetic. Most surfaces are more magnetic than a bottle cap, though, so if you want them to stay together, a spot of hot glue or super glue will be needed.
The magnet is thick enough that it protrudes some on a raw cap and a little on a traditional used cap, so the cap edges don't come in contact with your fridge.
Magnets ship next to the caps; you assemble as desired. Priced per magnet, caps not included.