I've achieved interesting results by custom-cutting a piece of insanely thick wall insulation foam board, and using it to replace the bottom piece in the box, then re-cutting the side pieces and leaving the top alone. The box has a lot less head-room, but it spent an hour outside at 12 F and held the temperature at 50 F inside the box. If the outside temp is over 30 F, instead of 12, we're back in a survivable temperature range. Best of all, it used one heat pack, and didn't overheat at room temperature.