I liked the idea of the pen holder clip, but I use a Pilot G2, which didn't fit into the hole, so I made a clip slightly larger to hold thicker pens. Edit 9/11: After printing this out and testing it, the hole was just slightly too small and kept pulling off the rubber grip. Added an extra 0.5-1mm, that way the entire pen can be pulled through and the pen clip can keep it from wiggling around.