I wanted to attach a bullet-style Space Pen to something, and decided having a cap that would fit the pen would be ideal. As a first step, I've modeled a replacement cap.
The original cap's inner diameter is 9.3mm, but when I printed a cap at that size the inner diameter was 0.15mm too small. So I made this one 9.45 mm ID, and it fits just right.
The shape could be tweaked to more closely resemble the original, with increased tapering, but I don't really require that at the moment and was more concerned that it would fit the pen full length.
Print with 0.4mm trace width to print the barrel with exactly two outlines. The tip is solid with a bridgeable bottom so it can be printed without support, but some infill is necessary.