A simple solution would be using a 555 timer to adjust the "duty cycle" of the resistance. The 555 timer would cycle the resistance element on and off through the solid state relay previously mentioned.You could use a potentiometer to adjust the duty cycle. It would be nice to have some feedback to tell what the temperature is, but thermocouples are expensive. If you don't have feedback, then the 555 is just as good as the Arduino and not nearly as expensive. I have used this simple circuit to control a ceramics kiln, and it worked just fine. The grunt work come finding what duty cycle corresponds to what actual temperature... have fun