Okay, I have changed my heat source ! Now I have one of those heat stripes attached to the button of the styrofoam box. I don't want to place my egg box right on the heat stripe, so I will add a styrofoam platform for the egg box to stand on. It is working pretty good even without a thermostat. It says about 35 degrees celsius (95 degrees fahrenheit) inside the egg box I will use, so I will have to cool it down a bit to 89 fahrenheit.

One fast question: where should I put the thermostat probe? On the heat stripe or inside the egg box?

Thanks guys !