A basking lamp, along with an under cage heater on one half of the cage generally works well. Make sure that there is a shady area as well. A good rule of thumb is to provide a 10 degree F variation within the enclosure. The ideal terrarium will contain at least two thermometers. When putting the heater under the substrate, make sure that there is no way for the lizard to get under the substrate and come in direct contact with the heater.
Savannah monitors have sharp claws, and become very strong.
Avoid using screen, because they can easily tear through it. If you use plexiglass, expect it to get scratches and need replacement eventually. The more their surroundings bore or discomfort them, the more they will try to escape. A good selection of hiding spots, placing their food in different locations, climbing branches and plenty of room can help in this regard.