For carrying pens, I like the pouch that comes with the ReliOn Micro because it is more rectangular-shaped than most of the meter pouches that you get, so it fits pens as well as your meter. The length of it is exactly big enough to fit a 6.5 inch pen.
It has a strip vial slot and a zippered compartment on the side.
If you are not going to use the meter it comes with, you can take out the plastic meter-holder and sew in a different holder, or just leave it out entirely.
I know that you were wondering about phone cases, but of all the cases I have looked at, when it comes to only carrying around a pen and BG meter, this one seems to be the best fit.