A slight haze last night with OAT of 43F.  Box was at 30.5F but no ice.  Seems I have gone as far as I can with my present setup.  Will try and construct a continuous black plastic bottom.  From the data it looks like rusted steel bight be my best bet.  Better conduction and almost the same emissivity as the black PVC.

I wonder if higher sides might allow better cooling?  The surface area of the base seems critical because when I tried dividing it in half, the results were worse.