I read a page on the moleskinerie recently that had a hack like this. The guy used a ribbon and cut a couple of slots into the ribbon threaded a pen into the slots and glued the ends and slipped it around the cover of his mole and voila! A nifty penholder. I made mine a little different; I used a 1 and a quarter stripe of leather, eyelets to attach and then threaded my pen through the open slot at the top.
It’s not very useful for me because I always keep my space pen in my pocket. I’m going to try and use it for some of my pens but I fits the space pen best, I’m going to try and make a couple of these larger for different sized pens.
They are not exactly easy to make but not hard either. Anyway, check them out.