Ya know, there are IIRC 3 surviving cows knees, one of which was on display at the Ft Pitt museum awhile back as part of their exibit on the American Rifleman.
The one I use is made of lightweight barktan, heavily greased. I roll it around my shotpouch strap and tie it there It is pretty much universal, works on everthing from my longland to my Rifle to my Trade Gun.