I use the fingers from rubber gloves. Cut off about 1 inch of finger and it will snap down over the muzzle, make a nice tight seal and it is thin enough that the bullet never even sees it. I use nitrile gloves, but latex would work fine too.

It is especially important for muzzleloading in the rain. I use this method and still hunted in a pouring rain for 10 hours with my CVA Bolt action and it fired instantly when I asked it to. Keep the muzzle up (to keep the action dry). I was thinking about how to make a rain cover for the action, but the 209 primer is hot enough to burn through trace moisture and seems to plug up the hole pretty well, keeping the big stuff out.

Just be sure to take the cover off when home, or water under the edge can cause rust.

Chubber