I regularly use a leather powder holder when I go muzzleloader hunting. A friend made it for me. It has a brass spring loaded top on it. I've used it for the past 10 years with no problems. I don't leave powder in it year round. Generally I fill it for the hunting season and empty in after. Never had a problem drawing moisture, of course I don't leave it out in the rain. As you probably know Oregon, where I live, is noted for rain. I wouldn't be afraid to use one for CAS shooting.
Deadeye Dick