What Roundball calls a " Frizzen cover" others call a "Hammer stall". It does cover the frizzen, so technically Roundball is correct. But, it also acts as a homemade " safety" by preventing the flint from actually contacting metal if the hammer is accidentally cocked and then released while moving through the woods, and brush. If the flint can't strike steel, it can't produce sparks, and without sparks, the priming powder is not going to ignite.
Hammer Stalls are neat little gadgets to make, and to use. As you can see in his photos, your hammer stall can be attached at the other end to your trigger guard, so that you don't lose it from your gun when you take it off quickly.
If you use a hammer stall in rainy or mist, remember that moisture can and does condense on the UNDERSIDE OF YOUR FLINT, as well as the underside of the jaws of the cock. Wipe those area dry from time to time, whenever you are drying the rest of the lock. Like Roundball, I tuck my gun under my arm when walking, and when sitting on a stand, ground or tree, with the barrel pointed down, so that any water that FALLS on the gun rolls off the muzzle, and not down into it. When its freezing cold out, I am more concerned with snow falling off overhead limbs, and trees and going down the muzzle, so I definitely don't point that gun barrel UPwards!
As you can see by looking at the "frizzen cover" or hammer stall, IT ONLY covers the vertical portion of the frizzen. If you are really looking to keep water out of the lock and breech end of your gun, then that "Cow's Knee" is what you want to use. Nothing about a cow's knee prevents it from being used With the frizzen cover in place, too.