For when you have a cartridge / pen insert of 3.20mm, but not anymore the "body" (or if you only buy the first).
Beside the voronoi idea, there is also the "laying flat" one that use the principle of showing the infill pattern (no top, no bottom, retract also during the infill, and choose a nice pattern, like "honeycomb").
Just need to design a cap now :)
The provided file may not suit all the existing ink cartridges (3.2mm diam. here), so here is the simple way I did this :
1) extrude an hexagone shape, 9mm diameter, for 100mm
2) with a hole in the center, 5mm diameter
3) use voronator.com to make it nice, I used the parameters "STL + less holes + thicker"
4) remove 0.3-0.5mm from the base to have a flat surface to add a brim to
By using "thicker" before, we have larger cross section surfaces (which is good to extrude at least something), but a smaller diameter : after being "voronoized" the base diameter of 5.00mm is roughly 3.80mm. That was ok for my 3.20mm cartridge.
In order to find the right diameter to draw I had to print and test several diameters, the 4th iteration was the good one.