This morning the "box" was 20 F with the ice frozen hard.  The outside temp was 35 F. with calm winds.  This 15 degree temp difference should get me close to my goal.

Recent trials included building a Styrofoam chimney around the box but this actually raised the interior temps above the outside air temps.  Adding a small amount of salt water brine to the pvc cap and using a small enamel cup for the distilled water definitely improved the ice formation.  An identical cup placed inside the box did not freeze.  The improved conduction must have overcome the phase change energy requirements. (I must credit my brother for this idea.)

Next step is to add some insulation to the outside of the box and raise the pvc cap up in the box to improve circulation.  Then I just need to wait for the perfect night with clear skies, low humidity, calm winds and the right temperature profile.

A fun experiment with no practical use...at least for now.