To get an even heat distribution at just the right temperature to easily pry the back off I used a steam iron, dry, set between wool and cotton. To hold down the phone I clamped it at the long edges with 2 strips of 1/4” furniture grade plywood cut at 7 degrees, screwed to 2’ 2x6; and then clamped the 2x6 to my work table. This made it much easier to persuade to back off without any oops!