A bin is good, if you have one big enough. Washable. When they outgrow the box you can cover your floor and put a wall around. I used plastic bins, boxes, etc. Anything they can't hop over or knock down. If your floor is carpeted, this may cause a problem if they can't stand on it. My floors are laminate, so very washable, but I still covered it. You can also use a floor lamp outside of the box/floor area, which I think is safer. After all, they are in the house, not a pole shed. Some people use a brooder heater which is on the floor and considered much safer.