I would at least get a styrofoam board (comes in 4x8 sheets at home depot) and cut a piece to fit inside the lid. Reason: Moisture sticks to cold surfaces. On sidewalls it slips down to the bottom of the hive and out. On top, it rains on bees. That plus your top entrance (which many northern keeps say is the only way their bees can exit for waste runs in deep snow) should keep your bees from being rained on with cold drops, and vented some via that top opening. You will want to insulate the nuc's lid as well, just a styrofoam sheet.