Probably the most appropriate style lockplate would be a round back plate, that is the L&R late English, which is available with a double throat cock, which would also be a plus. You could round the back of the others.
As Mr. Brooks said, if a half stock I'd do percussion (for which copies of Leman locks, both round and square end, are available), or conversely I'd build a flintlock full stock with a 39-43" barrel. Just my personal opinion.