As I posted before, I have six Frixion Ball Slim 038s in my purse pen case. I use these primarily in my planner for the obvious reasons: they are slim and colorful. I am a very visual person, so I like having my events and activities color-coded and the thin tip lets me write in those small calendar squares while the slim body lets me fit them in my pen case alongside the other pens.
Unfortunately, their slim bodies also meant I was constantly digging through my pen case to find the color I wanted. But instead of admitting defeat by buying another pen case - because "defeat" doesn't mean just dealing with it, it means giving myself a valid reason to buy yet another pen case - I got creative and made this case out of a plastic postcard sleeve, a blank business card, and some masking tape.
Not bad if I do say so myself. I've been using it for about two weeks now, and it works like a charm. I might make a bigger version for my other pens.