I held the blank cross ways in the smallest set of O'donnell jaws, turned about 3/4" to the diameter of the end of the pen, rounded the end off and then cut the bit that goes inside down to the internal diameter of the tube with the tip of the skew at about 1/4" long then parted off.
It was a tight fit when offering it up, about the same as an end cap, but as a precaution I did roll a drop of CA around the inside of the tube.
I thankfully had the foresight to quickly line the clip up before the glue took hold! 
