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There's no flies on you, Kevin - so far, so good.
For the plug, I trace the outline of the big end of the horn onto a 3/4" thick piece of wood and cut out the shape with a saber saw, just inside the tracing (to allow for the horn thickness).
Then I work down the diameter of the plug gradually with a sanding disc, then files, until it slides inside the horn at least 3/8" (+/-) - leaving the rest of the plug proud for the time being.
I epoxy & pin/peg the plug in place, before shaping the proud section with the sanding disc, then files, to a slight dome shape.
I you want a filler hole in the plug, either buy a ready-made brass plug from Track of the Wolf, or drill a hole in the center and ream it out for a tapered peg that had a knob left on the end for the powder horn strap.
A period correct rear strap holder is also easily made by bending a nail into a "U" shape, cutting off the ends evenly, then drilling two small blind holes to accept the "staple", which I also epoxy in place.
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