Homemade incubator's thermostat won't shut off

[TheEggFactory]

I just bought a Westinghouse Make a lamp kit (120v) along with a lower double pole 240v water heater thermostat. The lowest setting is about 100 degrees. It gets to 100 degrees, and it won't turn off the lamp! It got to its highest setting, but the lamp is still on. I've researched, and it said that 240v thermostats should work on 120v incubators. I think they only make 240v thermostats for an incubator unless its something super expensive. What am I doing wrong? Do I have to get a wafer thermostat? Please help.