You are way overthinking this matter.
Re-read bedbugbilly's post and take his excellent advice.
Sealing the butt plug/cap isn't rocket science. At events look at other folks horns and get some ideas. You can permanently fasten it in with a lot of things. I have used brass nails, locus thorns, toothpicks.
A big part of muzzle loading involves being inventive and doing for yourself.
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