I have a few fountain pen cases. Here are the two I use most.
One is a single Parker pen pouch, bought a few years ago for £1. I think its made of bonded leather or PU leather and it can carry the largest of pens (in this case my Jinhao 159, which is the same size as a Montblanc 149.
The other I use most often is an unbranded triple leather case which has a magnetic clip. The leather it is made from is very soft. This has dividers, but they only go down for about an inch. This fits slim or medium sized pens.