I make the night box lid out of half a sheet of plywood, so 4x4 feet. Then the box has to fit inside of that. With the wall thickness and spacing for the lid, it ends up being around 39x39 inches inside. I find that the 18x28 inch Kane mat and a 21 inch 80 watt radiant heat panel works perfectly. They can move off the heat mat when they are warm enough, but still be inside the heated box.@Tom I'm trying to prepare for Bumblebee (leopard tortoise) to be outside. He's almost 8 inches now. I know he's not ready yet and his growth is much slower than Optimus Prime. He may not be ready at all this year but just in case. What size Kane heat mat for a leopard tortoise?