... I may be doing a partial or complete "interior" rebuild of my 25 year old Frig Box, that actually (surprisingly) does pretty well keep in heat out and cold in. Other than custom vacuum panels, is Polyurethane Foam Insulation still the best practical insulation material available ?

I am thinking of using DOW blueboard, which I think is an extruded polystyrene. Not sure there is any significant difference, maybe the styrene will be better for moisture/condensation, but hopefully there won't be much if any, with proper vapor barriers. From DOW > Styrofoam Square Edge (extruded) - This product is often called "blueboard" due to its light blue color. In spite of its relatively modest "R" value (4.92 per inch), Styrofoam SM is our overwhelming recommendation for foam ice box insulation in marine applications. Unlike its expanded cousin (above) and virtually all other foam insulation, Styrofoam "Square Edge" is completely impervious to moisture. This is a huge plus.

thanks, cheers, John