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Reptiles are cold-blooded creatures, and they need to have a certain median heat around them at all times or else they will become cold and die quickly. Most reptile cages, except those in areas where the natural temperature never falls below about 70 degrees Fahrenheit, have a natural heating system, like a heat pad in a certain area of the cage so that if the temperature does fall, the reptile can stay warm on the heated area.
On a Portable Heater
If there is a portable heater in the house, a homemade heating pad can be created. With a metal table or shelf, the portable heater can be set on the metal shelf below the tank and turned on high. As the heat from the heater rises, the metal below the tank will become warm, in turn warming the tank itself. By taking towels and folding them under the tank, the heating pad can be created and regulated in degrees. Make sure the shelves are metal and there is nothing flammable on them as these portable heaters are known to start fires due to carelessness in being close to flammable materials.
Beside a Heating Source
If there is a central heat where you live, the reptile tank can simply be set in front of the heating vent and the heat turned on high in the house or apartment. This will warm the cage significantly and be directed solely at the tank by closing the other vents around the house, so that it doesn't heat the rest of the house unnecessarily. Another good idea is to leave the tank beside the exhaust vent of the dryer, if there is one in the house and running the dryer constantly. Though this is not good for the dryer and may cost a bit of money in electricity, it will keep the reptile cage warm.
By soaking towels in boiling water, these can be put over and under one side of the reptile tank and then covered with a thick dry towel. The heat in water sticks around for a while and stays even longer with the dry towel is placed on top of it, as it can't go anywhere but through the glass of the tank or the screen above it. This will effectively warm the tank and create condensation inside for a few hours, though these towels should be replaced regularly as the wetness of them can quickly turn from warm to cold. This may be a painstaking process, but hot towels will be effective for your reptile in the short term.