Cloth muzzle stoppers are specifically mentioned by Captain Thomas Simes: "A foraging cap and stopper too be made up, conformable to pattern ones, cut from part of the old coat..."1.
To make a muzzle stopper from and "old coat", cut a piece of coat wool about 4-5" square. For .75 calibre British Land Pattern (Brown Bess) muskets, you will need a .69 ball (the standard issued size). For other arms, find a ball about .05 or .06 smaller than your bore. If you are not a shooter or don't know anyone who is, a marble, ball bearing, or wooden ball could probably substitute.
Set the ball, sprue up, in the center of your wool square. Make an O.K. sign with your left hand and push the wool and the ball through the hole made by your finger and thumb. Stretch the wool around the ball as tightly as possible, gathering the excess wool up around the top in your left hand. Keep stretching and working the wool until there are no folds or creases around the sides of the ball. Next take a piece of good, stout thread or linen cord and tie off the wool tight to the ball. Wrap it around a few times and tie again.
You may want to mark the stopper with your rack number -- use black paint or a paint marker for this.