Good heat source for iguanas?

we have an enclosure that has the front an back made of gl, both sides made of thick wood, and a wood bottom. currently it is small but we are still buying the materials to make it bigger. there is gonna be three sections, the top which is what there is now, then there is a shelf which we are goona put gl on, and then go below that shelf to the bottom. we are going to saw a hole in the current floor to get to the second, and then another hole in the second floor so they can get to the third. we have two of them and we have an outside cage that is huge and only for the summer and warm days. the top of the cage inside is wire. they have a light for the day, but need something for night time that doesn't have light. should we use a heating pad or what? remember the floor is wood so we can't use an undertank heater. right now it is only getting to the 60's at night, but it will get much colder. we live in texas but it gets to the 30's and 40's in the winter. any suggestions please let me know!