I use a simple piece of leather drapped over the lock. It's heavily oiled so it won't soak up too much moisture. I simply flip it off just before the shot. You can also use toothpick and apply some crisco around the edge of the pan. This takes a little experimentation to get just right. Then I close the frizzen and apply a bit more crisco around the frizzen to barrel gap.
If it's raining Cats and Dogs, your frizzen and flint will get wet and may not spark. All part of the challenge.
