I store my pens in a kind of cardboard box with mini dividers inside. This is what it looks like empty:
The inspiration behind the dividers comes from Enkore, a makeup guru on Youtube. I used the pattern he uses to make his lipstick holders, and made 3 of them. I made 3 boxes for each set of dividers, and then fixed all three together in a row before covering it all in black card.
Having it divided it into 3 compartments stops it from warping too much. You can see even with the dividers its still a little distorted from the bulkiness of the pens.
This is how it looks with all my Promarkers inside (I keep the 6 duplicate colours I accidentally bought elsewhere as backup). I just store this on my desk so its easy to just reach for a colour whilst I'm working.
When I purchased my Letraset wallet, I also bought a pack of 'end dot colour labels' to label the end of the pens so its easier to see what I have. These cost just under £2, and I think worth the money. There are blank back-up labels in the pack too, should you lose a pen or accidentally (ahem) buy a colour you already had.