Opening oven on Shabbat

Question

We have a gas oven. We leave it on on Shabbat and make sure that the light stays on even if the door is closed. I was told long time ago that I should put a small stone, or similar object on the door to prevent it from closing properly. This way when I open the door (which was open a little bit to begin with), I would not be the main cause of the temperature droping and the gas igniting. Since ovens have a vent in the back, do I still need that stone in the door? Also, should I be waiting for the gas to turn on before I open the door?