The "Hold-Heet" glue pot is the standard type sold by the piano supply
houses.  It is not designed like a double boiler; in the "Hold-Heet"
pot water is added to the glue, but not heating unit.  A water jacket
(like on a double boiler) is designed to distribute the heat around
the glue.  See http://www.infinish.com/products/gluepots.html

The "Hold-Heet" pot is designed to electrically distribute heat to
the glue mixture without the water.  It might work with a water jacket,
but there's not much room for the water, maybe 1/16 inch.

Philip Jamison