Rabbits live outside all year in the wild, they just need to find shelter, holes underground, and brush piles in well covered areas are preferred. They like 60° weather down to 40°. Under 40° to 20° for short times they are ok if they have shelter that is protected by the wind. So it is totally fine to keep your rabbits outside, just give them protection from wind and cold. I have saved bubble wrap to enclose and insulate my hutch, other recommendations are tarps or blankets, but with blankets it is recommended to cover with plastic sheeting.
It is important to leave a area open so there is always fresh air. It is common sense to leave your door area open for easy access for feeding. I clipped the end with a reg office paper clip and stapled the wrap together to get the correct length. I doubled for extra protection and stapled the pieces together. Then tightly wrapped around and clipped again on the other side. I then wrapped the top of my hutch, I first covered the open door using a hand towel. I put a few blankets inside my hutch for the rabbits to nest in. Then wrapped the top with a simular method, with no wire I stappled the ends together and then stapled to lower wrap to keep it from blowing off.
I also will up my feed in the winter and add a 3 or 4 sunflower seeds π»when it will be extra cold. Sunflower seeds are great to help with heating their internal temperature. They will need the extra calories to keep warm.
I hope this helps you to have happy bunnies this winterπππ