Another alternative is felt wads in a straight case. Punch them out with a 5/8" arch punch or just grab up a bag of 1000 .58cal "wonder wads".
I got the Wonder Wads as they work equally well if I occasionally use black powder just for the original feel, smoke, and smell of that bygone era.

http://www.buffaloarms.com/Detail.aspx?PROD=163384&CAT=4115

Stack them in as needed to fill up the airspace.
- Mike