You can use flexwatt heat tape but you need to use a thermostat with it because it will get really hot. In an enclosure that big you may also need to use heat lamps in conjuction with the heat tape. What material are you using. By all means stay away from plywood if you can it has the potential of splintering and coming apart with moisture and also plywood is mostly made out of pine which is bad for reptiles. At lowes they sell Aspen Boards up to 1" thick i would use that. But the key to any reptile cage is to always create the proper heat gradient.

What do yo plan on keeping in this huge cage?