There is a lot of discussion on the OCC Copics for Card Makers forum on how to store Copics. This grid that I found at Crafter's Companion holds the markers horizontally and has been ideal for me. I have these stored directly above my craft desk so I can look up and pull the colors I want with ease. It's easy to read the numbers and select the colors I wish to use. I also have my Distress Markers stored alongside them plus some other often used marking pens like my white gel pens and a few PITT Pens for journaling.
Link to the Ultimate Pen Storage System: