I store my Copics horizontal too. I purchased a used cd display rack from a thrift shop in my town, went to Dollar Store and purchased for a $1 a foam core board. I cut squares to fit shelves and even have labels printed out with color codes. I love it and hardly cost a thing! Plus I can find what I have easily.
Thanks for the storage suggestion, Paige! I have an old audio cassette storage case somewhere in the basement. I think I will try to find it to see if the slots are wide enough for the Spectrum Noirs.