OK, so you hit a nerve less than a week after I signed up.
Being a Cordwainer ( shoemaker) I kind of backed into my choices of leather to keep my flints firm. Aside from the fact that I have a great rifle now, all my guns get anywhere from fifty to maybe eighty pulls out of a flint and up to a hundred when I knapp & tap in the jaws.
Leather will shape and mold itself far better than lead with less muscle. Unless, of course you're a lead guy, in which case I get accused of blowing smoke.
Anyway, here's how I do it. I'll take a scrap of thick deerskin or elk...not the commercial split stuff but the seconds and discards that several vendors show up at rendezvous with because it's irregularities are so much better than the milled and split and curried uniform stuff off the retail shelf.
If you gotta use cowhide, take it off the belly where it stretches. Look for thr frizzy, soft ends that won't work on any other project. You don't want it too thick,so five ounce is OK. Stiff is bad, pliable is good. Cut strips about as wide as your flint. starting at one end, fold the strip over the flint so that it barely clears the flat or the top ridge of the flint and barely intrudes on the taper of the flint. Eyeball it cause you don't need the caliper.That's a good length to cut...just a whisker too long top and bottom.
Take a hole punch about as wide as your top jaw screw and punch dead center on the leather. When you seat the flint, push it as far back as you can and make sure the flint edge evenly lines up with the frizzen face.
When you seat the flint, if you gotta tighten so much that the top jaw starts to angle, then the flint is too thick OR you did not punch the hole in the leather. I get fussy and pick out flints that have a flat behind the bevel, but that's just me.
I have found that, after about thirty shots or so, when my flint has been seated a while and lived through lock cleaning, a tap or two with my brass knapper while the flint is seated will double its life. The leather will mold to the flint and cock jaws this way.
Hope this helps
The Capgun Kid