Ok, then one last question about this box...How tightly should it fit to the nucs? Should I leave a gap? And also, wouldn't this trap moisture inside the nucs (or does the open bottom provide sufficient ventilation)?
It is good if nucs are side to side. So they give warm. Insulated walls are ofcourse the best protection to bees.
My brother nurses bees near polar circle in Sweden. He put his bee hive into a bigger hive and put insulation (newspaper) between hive hive and summer hive. Bees managed fne 20 years and then he stopped whole beekeeping.
Of course the big box is colder than nucs. So moisture is ventilated outside and it condensates on the walls or just in "cold air" when warm moist air meets the colder air. Open bottom keeps nucs dry. One trick woud be the best and simplest:
You put your two nucs sice by side outdoors. One entrance to west, and one to east.
Make from styrofoam board extra walls and keep entrance free bees to fly.
Make a little upper entrance that air circulates out.
It means that you make a bigger hive, when you put double hive inside. - Good insulations aroud that double hive.
This will help colonies in early spring when they start brooding.