I've seen a number of solutions out there for storing Copic markers. I love baskets and just needed to section off compartments. Here's what I came up with.
I cut foam core board into a grid. The trick is to use a sharp Exacto blade and a quilting ruler.
This works for upright or vertical storage. And as for the basket, a basket with squared off sides works best, but other than that, you can customize the grid. I've been using this for about a year now and haven't come up with a better solution.