The wind finally calmed and the OAT was 36 F so I assumed the ice would be well frozen.  The water in the PVC cap was liquid but the box was 21.5F.  I put the temperature probe in the water and it caused slushy ice to form immediately.  Seems I had made super-cooled water.

Tonight I am going to try a copper plate with a center well which is painted flat black.  The shiny copper was a complete failure.