I have a snuggle safe that I use on my rabbits. I kind of have a rule that if it goes to below 0 I get the snuggle safe out. If it goes below -5 or -6 they come inside, but just for the night. If it goes like it did last year (it got to -25) they come in 24/7 I think they were in last year for about 4 weeks all in all, I just make sure that when they go back outside, I put them out in the day only first, then they adjust to nighttime too. I just pack their hutch with tonnes of straw, put on the homemade shutters and they snuggle together anyways, most of the time, the hutch is that warm, that when I open the top, I can feel the heat inside.
This year I'm looking to make a homemade bottle Snuggy contraption as I brought a thermal covering but they don't understand that it's still their water bottle and they don't drink anything. I'm in a attch a box to my shutter with loads of insulation in it so the bottle fits inside and you can still see the whole bottle from the inside of the hutch.