105 degrees is enough to burn a snake given enough time laying on it. You absolutely need a thermostat not a dimmer to make sure the temperature on the inside of the glass does not get over 92 degrees to prevent overheating. The top of the substrate does not matter, only the inside of the glass as the ball python will burrow or push substrate to get down to the bottom.

When you get the thermostat the probe goes between the UTH and the glass on the outside of the tank, not the inside.