The UTH or flexwatt can go directly on the bottom of the tub as long as it is regulated with a thermostat or rheostat. No need to worry about raising it up or anything. Just make sure it doesn't get above the 85 or so safe hot side temp for corn snakes and everything will be fine.
Before I got my rack on the tubs I used, I would tape the flexwatt direvtly to the bottom of the tub. Seems like trying to raise it up on blocks would be a pain