Mike, that might work. When used in a factory it's carefully controlled by humidity and temperature so as not to leave voids , but still does. What I did was discard the original box and fabricated a new one that was square, which would allow the use of material similar to Blue Board, although we used vacuum panels which are no longer available.
Foil won't retain heat and has no R value. The theory is that UV radiation penetrates the box and adds to the heat load, foil is suppose to reflect the UV. As I said ASHRAE (American Society of Heating, Refrigeration, Air Conditioning Engineers) has done studies and they were inconclusive, but I would add foil, it can't hurt.
Two other areas are the door gasket and drain. If the drain is open and a poor door seal, heat will thermo siphon and add a tremendous heat load. We have a double gasket and eliminated the drain. The performance of the system is more dependent on the box then the mechanical equipment.
Jim