You would need to use a reptile thermostat. A good reliable one for multiple cages can be found at reptilebasics.com. You just put the thermostat probe right on the flexwatt tape and set the thermostat to the temperature you want. Make sure to put a thermometer inside the cage to measure the temps your snake will be experiencing.
If you are going with custom stacking enclosures, you might want to look into Radiant Heat Panels: http://www.reptilebasics.com/radiant...panel-faq.html