I got some bottle top magnets for Christmas a few years ago that I really liked. I'm not sure why it took me this long to do it myself but I finally did. Here's how I did it.
Mod Podge Dimensional Magic
then used that to cut out more cardboard and patterned paper pieces. Make one cardboard piece and one patterned paper cutout for each bottle cap you have.
I used two glue dots stacked on top of each other in the center of the inside of the bottle cap and then pressed the cardboard on top of that.
I put another glue dot in the center of the cardboard.
Then pressed the patterned paper at the top of that.
Coat the top of the patterned paper with a layer of Mod Podge's Dimensional Magic. I did about an 1/8 of inch layer, let it dry for 3 hours and then because I wanted to make the bottle cap look more full, I did another layer. Make sure to get rid of any bubbles, a tooth pick or safety pin is a good way to poke them out. If you wanted you could also just stack up more cardboard paper and use less Dimensional Magic.
After it totally dries, I used E9000 glue to attach the magnet.
Voila, it's complete. These are really quick (minus the waiting time), easy, and cheap to make.