Z....IMO, and FWIW, I would make sure I was going to need, and use one before doing a lot of anything.
I have owned a couple over the years and they were just more hassle that anything much of real value, and that was when I lived in Shelton, WA the real "back door" to the Rain Forest.
Tucking the lock of my flinter under my arm proved much more practical that trying to cover, and uncover, the lock......besides, when it's pouring down rain you need to freshen the prime about every hour or so anyway...just dump it out, wipe / dry the pan, and re-charge.....IMHO, a "stall" for the frizzen has a lot more value than a Cows Knee.
Still yet, all this ramblin is just one man's thinking. If you feel you need a Cows Knee, then go for it. A lot of folks do have 'em and use 'em, so they can't be a real bad thing.
Uncle Russ...