Extra insulation is hard, because most units these days use the steel case as condenser, with steel coils glued to the steel sheet, held down with tape, before the insulation is foamed in place. Thus no coil on the back, and the interior coils are the same, placed on the plastic case to cool that way. no way to insulate, though the first run of the hot gas out of the compressor does run around the door frame, so that the seals do not freeze onto the metal. Best is to not open during power failures, and for longer use a chest freezer is good, and many also have enough control range to act as a fridge as well. Yes the fill with ice works, a lot of energy stored in that phase change.. BVut for longer than 8 hours you probably want a generator, or inverter and battery, charged from some source, solar, generator or even that pedal bike, just needs to have enough energy put in to run the compressor for an hour a day.
Tip for the freezer is to take a small plastic bottle, fill with water half way, and freeze. Then remove, open, and place a coin on top of the ice, close, and place back inside. That will tell you that, since you last looked at it, that there was a power failure long enough for the ice to melt, as the coin will be in the ice, and not on top.