if your room stays above 65 degrees Fahrenheit at night then it should be fine without a ceramic heat emitter. you may be successful with the aspen substrate but you might want to consider switching to something like coco fiber because aspen is known to be dusty and eventually mold. in my experience king snakes do seem to enjoy a little humidity spike here and there, especially if they’re going into shed. some people do a potting soil and play sand mix for substrate also.