Winter boondock... Cold cab over. Need ideas.

I would think 1/2 foam would do well for the windows. Use blue painters tape to hold it in place. I use the tape to hold in acrylic I use for storm windows and don't have problems removing it after the winter. I just make sure it only adheres to the metal window frame, not the wall itself as it will peel off the paper after a few days.

An alternative would be to hang a heavy curtain across the cab over opening trying to keep the cold in that area.