I put a leather buttplate on a Yellow Boy Rifle I used in Cowboy action and a musket I owned about thirty years ago. In both cases I wanted a shorter pull and did not have the talent to adequately replace the buttplates. In both cases I used some sole leather I had in inventory and a bunch of Contact Cement and shoe pegs. Both of them held up well for the two years I owned and used the pieces. In both cases the trigger pull and lack of talent/money were the factors. I did not take any prisoners when I handled my musket on cement firing lines and dropped the butt to the ground. The Yellow Boy showed almost no wear until the gap up at the bolt got a little large. It only shot black powder and I traded it in with a clean looking leather butt plate.
If you end up using leather, make sure you gouge ( skive) the fold on the bend at the point of the comb so that the bend has an even fit and covers all the wood tightly. Treat it like a shoe sole by wetting it, shaving the flesh side edges for a curved fit and molding it with hammer taps over the entire gun butt. Put some contact cement on the wood, and apply a piece of Saran Wrap spacer while the glue is wet keeping the water off the wood. Then coat the saran again with glue and apply the leather while the glue is still wet and tap the leather tight. I let both dry in position the way I would do with an inner sole. The next morning after peeling away the Saran, slopping on the contact cement to both wood and leather and letting it dry I gingerly fit the leather to the wood. I tapped the arrangement until I knew I had a tight fit I couldn't use a pegging awl, so I took a drill bit half the diameter of the shoe peg and spaced the holes about a half inch apart. Push the points into the hole and pop pegs in with only one shot of the hammer or they will shatter. When wet they will swell in their holes and seal them.
The contact cement will help seal the wood, and a boatload of oil will help seal the leather
Closely trim and then burnish the edges of the leather with water, beeswax and a boatload of brisk rubbing with a glass jar until it squeaks.
Knowing what I know about Cordwainers, if any of them applied leather butt plates back then this is the way I'd bet they did it, only they had to use paste. Like every other leather product, survivors would be undocumented and hard to find. I am sure there must have been some Joe running around in the woods with this kind of arrangement
Don't shoot yore eye out, kid
The Capgun Kid