A uth will provide a concentrated hot spot. That hot spot needs to be big enough for the animal to fit it's body on, but small enough to allow for the snake to seek out cooler temps.
A standard 10 gallon aquarium measures roughly 20 by 10 inches. If your tank is that size, I would recommend an ultratherm or cobra heat pad (same thing) that measures 6 x 11 inches. That should give you a 6" strip of heat at one end of the tank.
As for placing it in or out of the tank - an ultratherm is laminated and can be placed in the tank. However this size is nominally wider than your tank so that may prove to be difficult.
The cobra mats also come in a 4x5 inch size. You could also buy two of those, mount them side by side, and provide an 8" wide strip which would neatly fit in the footprint of the tank.