I haven't mastered freezing in mason jars. Thus I freeze in plastic bags still. Its not ideal, but it works well, and I actually use the stock this way, and its pretty much the only thing I use plastic for.
I cool the stock completely (refrigerate it until cold), then pour stock (either reduced or not) into plastic ziplock bags (usually 1 qt). I freeze them flat on a cookie sheet, then they can be stacked or put together in a box for freezer organization. Its pretty compact. I like 1 qt baggies, because its a good quantity for using. If I need less, about 1 minute under cool/room temp water and the stock becomes "soft" enough to break off chunks, without actually defrosting. I don't like using plastic, but it is a very very easy effective method of storage.