You can't use bedding with a heat mat on the floor. Its not safe.Seems I finished just in time as temperature this morning was 51 degree F. He is snug as a bug in his mulch in his new house with the oil filled radiator keeping it warm. His heat mat is in the mail. I used the reptile forest floor mulch but am looking for some more cheaper...mulch that can be added to this. I'm also going to order a thermometer to monitor the temperature inside the house. First I had it too warm then adjusted it felt too cold and last night got it to where it seemed ok. I still have to add his light as its pretty dark in there and for days he doesn't want to come outside. I have a shower curtain, never used lying about and plan on using that on the door so it can hold some heat in while giving him access to the outdoors. (I'll cut strips in it as I can't find any of the flooring type. I did see on line they sell strips the are used on commercial refrigerators but man they come in big rolls and are very expensive. But I think they would be ideal. Maybe I'll buy a lottery ticket and see? lol Yes Tom it is a journey . This box is about 4 ft high enough I can get in and clean and make heat adjustments and not enough not to break my old back climbing in it. My back and sciatic nerve is out of whack today so its a rest day. My recommendation is to go with your 2 ft box with hinged roof, too late for me but good for others.
Don't go by how it "feels" in there. Mount a thermometer or two to the walls. I like to have a thermometer with a remote probe. I leave the probe in the box, and can monitor temps at any time from inside the comfort of my own "night box".
You can buy the freezer door flaps by the foot, or in smaller sections. It is MUCH more effective than other materials. Overlap it at least an inch per flap. I can't find a link for anything less than a 100 foot roll anymore. I bought a 50 foot roll recently and I can sell you some of mine if you want.