Mark- No rubber plug. Plastic yes, rubber no. It will eat the
silver. Also you will need to gold plate the interior and the cap of
the salt shaker as salt eats into silver as well.
You are right about the larger threads being die struck. It’s easy
to make a simple twist and lock mechanism. The top has two bumps on
the bottom edge and the the base at the top has a rim with two slots
that the top fits into and then is twisted to lock it into place. A
hinge is also a dandy way to solve your problem as well.
As for holes it depends on how much you like salt and how much your
doctor allows you to eat. Sigh. Also I like my pepper coarsely
ground, some like fine ground, so make the holes the size you like
yours to have.
Post photos when you’re done.
Have fun and make lots of jewelry.