Can't remember where, but I've seen a modern-made, Western-style boot with a sheath stitched to the inside near the top. It seems to me that there were enough people with enough talent at working leather that a man with enough brains to think of such a thing could describe it to a man with enough ability to make it. I figure we ain't no smarter than they were and they could do cool things with leather too, so this would fall under the "makes sense even if we can't find a picture of it" umbrella.
And no, that umbrella does not cover shotgun shell belt slides, because they didn't make no sense nohow until $A$$ came along.
Ding! Just remembered where I saw it: The Leather Shop, Centreville Road, Manassas, Virginia, circa 1995. If you're in the neighborhood and it's still there, walk in and buy something, they're good folks and they always did right by me.