Me personally, I would use a ceramic heat emitter of around 100w and a thermostat to control temps. You could play with the placement a bit to see if it helps humidity keeping it over the water feature and so on. As for keeping the humidity, cover every square inch of screen or get rid of the screen completely. It's a snake, not a hamster, and doesn't need large amounts of ventilation to be healthy. You can uncover screen or add holes as need to maintain proper humidity.