As with the holster for a handgun, there are options:
[1] Soak the holster with neetsfoot oil.
[2] Soak the case or holster in warm water until it is soft and plyable. Put the magazine in a plastic bag and let it dry.
[3] Get a cloth magazine pouch.
[4] Go to a knife shop and get a folding knife pouch that fits the magazine.