Approximately 1/8" steel or brass would work -- thick enough to take the battering that a buttplate often does, but still bendable. Also, given that your stock has a modern appearance to it -- despite obviously having a sidelock -- you might try Brownell's, Numrich, or Dixie Gun Works for hard rubber or metal buttplates as used on cartridge rifles of one sort or another. They generally provide the dimensions of the plates they sell, so you could find one to match (or be ground to match) your stock's profile.
By the way, there's a "Markland barrel" on eBay that was made in Spain -- 33.5" x 7/8", .45 caliber. Markland was yet another of the distributors, like Jukar, CVA, and Traditions, that marketed essentially the same "Kentucky" rifles over the years. If you haven't found a barrel yet for your Jukar, you might take a look.