If you're using a tank, or even if you aren't, eco-earth substrate, coco chips, husk, or orchid bark are usually recommended. They hold moisture better, which helps to keep your humidity up. Aspen is okay but can be a pain in regards to humidity, which is already kind of a pain to manage with a tank.
Cover the sides and back of your tank, it will make your bp feel more secure. When I used tanks back in the day, I used black paper. Do not tape it on the inside of the tank. Tape can cause some really nasty and sometimes fatal injuries to a snake.
Okay, thank you, can i use white paper? Like printer paper? Ans yes I know not to put tape in the tank.