This happened to me last year. It was April, and the worst of winter shouldve been over, but on day 10 or something, it was like -28C. So I squished the cars up together in the garage, got a big appliance box, filled the bottom 8 inches with shavings and paper and then straw on top (what she is used to in the barn). I cut a large chunk out of the sides and top for breathing holes(sometimes i'd leave one flap open on "warmer" days....
Anyways, in the large box, i put a smaller cardboard box to replicate the size of the nesting box.I just got a 14x 14 box , taped the top closed, and cut out one side. Like a dog house. I had my husband help carry the eggs, i carried her and we transported them one evening. She was in a trance basically, so no biggie. I recall she walked around the new box clucking for a bit, and then settled down on her nest and that was it. I was very glad, too- because temperatures stayed really frigid and the day they hatched we had an ice storm!!!! The chicks were happy in the appliance box with mom for about 2 weeks, then I moved them (box and all) into the barn... then about 4 weeks she would mother them during the day but roost at night- so it was nice they had eachother (9 of them) and they would cuddle in the nest box without her, then some started to roost with her under her belly at 4 weeks or so.
Good luck!