How to control temperature in an experiment?

Look for heating strips that have a thermostat to control the temperature.  

Even better, if it's affordable, is a thermistor heating device.  It self-regulates and doesn't require a thermostat, which is one more thing that could fail over time.

But the simplest answer I can think of is as follows:
- Leave the room temp seeds on a counter out of the sunlight.  The closer to your room thermostat the better.
- Put the cold seeds in a refrigerator (which is using a thermostat)
- The most steady heat source that comes to mind is the exterior of a freezer.  

With all of these locations, monitor the temperature over time before you make your choice.