i personally wouldn't use cardboard at all bc it deteriorates fast and is prone to mold build up. Heat resistant plastic, wood (not pine), and ceramic hides all work great. Make sure that there are hides on both ends.
Also at that size and age.... move up a size in the tank. They need a tank that's about as long as they are. 30-40g will be fine.
Make sure that the humidity stays stays around 65-70%.
lastly, you really should get a thermostat that controls the temperature, especially on the warm side.
I can go on and on about the basic requirements so feel free to ask anything that you are unsure of. There are also a lot of great threads on here that will help you out if you search. Best of luck and let me know if I can help anymore!!