If the salt cellars had plastic caps with knobs on the inside that would just fit into the holes of the shaker, that would solve the (moisture/clogging)problem.
Don't rush to patent it, Dr Bill. Many cafés (naff caffs) in Europe use salt cellars with a hinged plastic top that do precisely this.

To me, a salt cellar is a table item, either as a shaker, grinder, or spooner. In our family we also have salt jars or salt caddies which are kitchen items with larger amounts of salt.
Rod