What exactly do you mean all the hotspots? Do you mean to control the warm spot in each slot of the rack?
The rack you are building, is it going to be 1 long strip of flexwatt going through each slot in the rack? If so you just need a thermostat with 1 probe, make sure you secure the probe well to the flexwatt, if it comes detached from the flexwatt the thermostat will crank up the heat since the probe is taking the temperature of the air above.
I would recommend either a Reptile Basics Vivarium http://www.reptilebasics.com/thermostats
Or the Herpstat http://www.spyderrobotics.com/products/herpstat.html
What kind of fail safe do you mean?