The Skadis containers are relatively shallow. This is a downside when using them as pen containers, because pens will get disorganized and hang over the sides.

By adding a container divider, pens will stay grouped and not go wide.

The amount of rows and columns, and the height of the model can be customized.
If the model doesn't fit due to revisions IKEA has made to the container, the width, depth and corner radius can be tweaked as well.

Due to the geometry of the container (the rolled rim makes the opening about 1mm narrower than the inside of the container itself), the insert might be a bit loose. A bit of double-sided tape on the bottom mitigates this (thanks for the tip @ms_davina_a!)