Temperature control experiments

I tried making a temp controlled box for yogurt making or egg incubating.  The goal is to come up with a cheap way to make a box hold about 100F.  I tried with a water heater thermostat and had some luck, but the temperature swing was about 80 to 125 F.  It would probably be better in a system with a longer time constant so the insides of the thermostat could stay closer to equilibrium with the box being controlled.
Then I tried the industrial controller.  It cost about $50 new and has a self tuning PID control algorithm.  It maintains the temp to within a degree or two.  However, the type K thermocouple seems to read different from the others.  In fact, all 3 seem to read a few degrees different.  So now we are trying to figure out how to get a real accurate temperature reference to calibrate to.