I have a big tank that I used to use for my showcase tank - it was heated underneath by a couple of heat pads but I also ran heat rope under the substrate to help heat the tank. You might also consider insulating the tank as well (back and sides) to keep the heat in. Cover the top I think has already been suggested. You can also attach those heat pads onto the side of the tank to help with ambient temps if you need to. Flexwawatt can also be used instead of heat pads. Be sure you put your heating devices on a thermostat.

Good luck.