Its fine to use heat tape if you want to.
Put your flexwatt on the side of the cooler. Then line the bottom of the cooler with water bottles to conduct and retain heat. Then place a light diffuser on top of the water bottles. The light diffuser acts like a solid floor for your egg tub. If your cooler is large, you can put a small computer fan to circulate the air and keep the heat even to prevent hot pockets.
There's a good tutorial in the DIY subforum.
Sent from my DROID RAZR using Tapatalk 2