An UTH (under tank heater) has adhiesive to mount to the external side of the tank, which I recommend for a corn snake because they need belly heat not air temperature. A basting light warms the air inside the tank.
Some people on here say that it is best to put a peice of tile or another peice of glass between the UTH and the tank. This not only disapates some of the heat (an UTH will get over 100 degrees by itself) but it also alows you to move the UTH to a different tank if you ever need to. I find that sticking the UTH directly to the bottom of the tank is fine as long as you have a way to control the tempurature.