Once the adhesive wears out I use low tac artists tape or hot glue. I want to to know the hottest point the snake can get to at all times. If I do not constantly monitor the temp than it can get dangerously hot and I would have no clue.
The heat loss from holes on the side of a tub is negligible. The most heat is lost from the top, not the sides. Covering the holes will have little impact on the ambient temperatures.