This is a tiny plug adapter that allows you to easily fit a Pilot G2 Gel refill into a Lamy 2000 Rollerball. The Lamy refills are both harder to find also limited to only medium and broad width. It's not rocket science but this little adapter pops on the end and catches the tiny teeth inside the Lamy pen. I left the centre hollow so that it can hooked and removed easily if it sticks in the end. I also suggest a quick drop of glue to affix it to the refill. 

I printed it at a max of .2 - material irrelevant. Just make sure that you have decent adhesion as it is small enough. 

I am using a Pilot BLS-G2-7-8 refill in place of the Lamy M36.