I have seen people use this:
For me, it's not big enough.
I love how this lady made her own marker storage from foam board:
Syzygy of Me: Foam Board Copic Storage
This is another idea:
Copic Marker Organization
Using a drain spout:
The Inking spot of Crain Creations: My Copic Storage...
I use something like this....love that it's portable:
To Cut or Color: Copic Marker Storage