Thanks for the suggestions. I'd like to try to build something my teenage daughter would enjoy shooting. We've already built a Lyman GPR together but it is too heavy for her to handle comfortably.
The hooked breech makes the GPR easy to clean (because she does that too). And she does prefer percussion over flint.
So I'm considering a gun in or around .40 with a 7/8" or 15/16" barrel at about 32-36" long. And yes, half stock too.
Also, I've built a TOW fusil, a North Star West NW gun, a Kibler Colonial Rifle, and half a dozen commercial kits. I'd like something that presents an additional level of challenge.
Don Stith's Squirrel Rifles may fit the bill. But I am wondering if there are other options out there that I can compare.