We just built an outdoor enclosure for our Sulcata, Nugget. This is his first winter outdoors. It is a 3ftx3ft enclosure/doghouse. The top is hinged so we can open and clean it. We have insulated the top and will put a carpet flap or possibly a door over the entrance in the winter to keep the heat in. We have a heating pad (Osborne industries) for the floor with a temp regulator. We are wondering what kind of substrate, if any to put in there? We live near Sacramento and can get down to 40s (rarely 30s) off and on during the winter months. Thanks any suggestions would be appreciated.