I put 3 or 4 layers of cardbord in the bottom and 2 layers in the sides and top of inclosed nest boxes for winter, --I roll the cardbord for the top and sides into a roll and push it into the box and then open it ,so the sides and top are made from one piece of cardbord, then I put the pieces for the bottom in and push them down between the sides so they fit snugly, that way it all stays put ,and the does don't mess it all up and get it out of place when nest building,-I check them each day to make sure there is plenty of shavings, and it is dry, I also make sure there is at least a couple of inches of fur over the kits, if not I pull more from the doe, or use some of the extra I have saved. [you can heat "clean" used fur in the oven to kill germs, [you will know right away from the smell if you get it too hot] I also make sure the opening of the nest box is not facing a draft [my pens are closed on 3 sides, open on the front, so I don't have the nest box opening facing the front] sometimes if it is going to be way below 0 ,I hang curtains on the front of the cages. --I don't bring kits indoors, I have no time or space for them -and-- I do not often loose the kits to cold.