Remove the top section of the carton, as you'll only need the egg-formed section for this storage solution, then place it on your fridge's door on one of the provided shelves.
Since these shelves are designed for eggs or egg containers, they'll fit perfectly. Now, turn the condiment bottles upside-down to secure them in the egg-shaped form of the carton.
You'll find that the egg shape offers a secure fit for bottle caps, as the cap's round shape sits snugly in it. It's held there by the pressure of the carton holding its shape.