Muslin casings can be purchased or homemade and are sometimes used with summer sausage and salami.
To make your own:
You will need a piece of unbleached muslin about 12" long and 8" wide. Fold the muslin lengthwise and tightly stitch a seam across one of the short ends and continue along the open side. Keep the stitching about an eight of an inch from the edge of the material. The short side of the seam can be curved in a
semi-circle to give the finished product a rounded end.
Turn the casing inside out so that the stitching is on the inside. Set it aside until you are ready to stuff it.
See Diagrams (Click on diagrams for a larger picture)
Info from: Home Sausage Making by Charles G. Reavis, Garden Way Publishing ISBN 0-88266-477-8