We built our own 2 cu ft box. (took 2 people all summer)...
We used "Vacu-panel" Vacuum panels. (Each custom made) The 1" thick panels = 5" of foam. By adding a layer of foam on each side, it = 7" of insilation! By using the vacuum panels, the outside of the box's dimensions were kept within what we had to work
with. With the box being small, super insulated, super sealed, 100% vapor proof inside & out, and foil covered on the outside... It only uses 25 or 30 AH / day. (IF we don't make ice)
spent on conservation of energy goes much further than money
spent on energy production.