Rollerball Pen Refill Case

My daily carry rollerball pen's cartridge is due to be replaced soon and I didn't want to carry the refill loose in my bag for fear of damaging it/exploding it all over the inside of my bag. Should fit any refill with a 6.3 mm diameter (0.25 inch) and a LOA ~112 mm (4.4 inch), including a Pilot G2 refill. Friction should keep the cap attached, but if not there is a provision in the bottom of the case for a small spring to keep it shut. The cap was printed without support, open end facing up w