Pilot G2 Pen - $1ea. (find here)
Montblanc Rollerball Refill - $6 ea.
(find here and here)
Hacksaw (like this one)
Instructions: I won't take any credit in this hack since I got the full instruction originally on Instructables. The original instruction used a G2 Pro, but one commenter suggested using even the cheaper G2 (w/ a full clear tubing) for a better looking outcome. I decided to test out using the clear G2 for myself since I had some refills around.
Result: It's a pretty cool hack. I do have a nice Montblanc pen but I hardly ever use it because I am always losing my pens. With this hack, I have the benefit of using Montblanc's high quality refill in a pretty cool looking display pen case. The hack itself and how easy to implement makes it a cool project!