+1 for a cow's knee. They work well for any sidelock, flint or percussion. Sabotloaders' pattern is good. I usually roll the leather over the front of the lock to keep from having two seams, but that's just me. Stitch it tight, grease it up, and it'll stay pretty darn waterproof. I also tend to keep the barrel pointed down and whole lock area under my right armpit (right handed) in any rain or mist. Keep the cow's knee on or lock under your armpit until the last minute before you shoot. Many guns have a "watertight" frizzen, though not really water tight. A little bit of stiff patch lube or perhaps gun grease (just a bit though) around the edge of the frizzen or at least along the barrel edge of the frizzen helps to seal it and keep water from channeling onto/into it.
Definitely use the flintlock!