Several years ago we covered the minimum number of coins you'd need to produce exact change for any occasion (10 coins), but we didn't have any great suggestions of how to carry them, but thanks to Instructables, you can cut a credit-card sized piece of cardboard to hold all ten coins in just a few minutes.
Instructables user RTourn found that standard shipping-box cardboard is thick enough to hold two coins by friction. He cut out a credit-card sized piece of cardboard and cut openings for each of the four types of coins.
Once this is done you'll have a simple and free way to carry exact change wherever you go. If rocking a cardboard cutout is too ghetto for you and you have access to a 3D printer, you can print out a plastic version of this change card.