I found DraftYeti's Parametric Pen Holder and thought it would make a perfect insert for cigar boxes to store fountain pens in. With a little tweaking, it's possible to size these perfectly to different boxes. I was given three boxes by a friend to start with, and wanted to provide ready to slice STL files for those who may not be comfortable making the changes in Fusion. 

I was able to print all of these on a Prusa Mini, I printed with a .6 nozzle in 3D Fuel PLA with .3 layers, but anything you're structurally comfortable with should work fine, there are no special requirements, and they print on edge without supports.

As I accumulate more boxes to work with, I will likely upload more .STL files, but will start with these three.