I would put a board over the cage to make a roof. I might use a shower curtain to protect it from the wind, and fill the hutch with plenty of hay. It looks like the new part has a glass front and back. I would probably drill some holes in the wood for some ventilation. It looks like Thumper is going to have room to exercise this winter, which should make him pretty happy. The
Humane Society used to recommend (before they formed a partnership with the HRS) that hutches should be six feet long. I think you did a great job on that hutch.