As I could not find a Gridfinity pen holder that fits an Edding 300 or other ones with a diameter ~17,1mm I created one with the help of Blender (for the four cutouts) and the Gridfinity generator at https://gridfinitygenerator.com/cutout

The used magnet hole size is 6.2mm (diameter) x 2.2mm (height).

Hint: Due to the limited space of a 1x1 Gridfinity container the walls in between are missing but it still works fine.

Recommendation: The magnet holes might need manual cleaning effort when using the version with the screw holes (at least they did for me), so if you do not need the screw holes please use the version “Gridfinity_Edding_300_4x_Magnet_holes_6.2x2.2”.