Tin foil is better because it's not permeable and it's not flammable. You can put it on and wrap it around the sides and it will stay put when you open the lid. You probably know this, but do NOT use tape to secure it on the inside of the lid. Never use tape anywhere your snake can ever get at it. If you need thermal insulation in addition to trapping humidity, a towel or blanket over top of the foil will help with that. For a more permanent solution you can cut a piece of foam board to the size you need.