I live in Ontario Canada,we make our boxes out of plywood with vents that can be closed and fill them with straw.When we go to Iows to hunt we put a heavy blanket or two over the top of the box-leaving the front opehn for ventilation.The box is a 2 dog box and it is in the back of a pick-up with a topper on it,have never had a problem with the dogs being cold