A economy rack isn't going to work for you, and just trying to add a second strip of flexwatt isn't going to do enough to fix your issue. Flexwatt does not greatly change the air temperature in the tubs. even if you can get part of the tub floor to 90 degrees, and the rest to 80 degrees the air temperature in the tubs is going to be too cold. If you are really wanting a rack, you will need to heat the room that it is in with something like an oil filled heater to at least 75 degrees.
Your best bet would be to get a PVC cage (I prefer the Animal Plastics T8 and heat the cage with a Radiant Heat Panel)
Here is a thread I wrote on PVC cages:http://ball-pythons.net/forums/showt...With-Pictures!