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.