Test it out for a few days and find out. I always like to go overboard with the heat source, just because it gets up to temperature faster if you open it up or there are any fluctuations. If you set it up in a room that is as cold as the area is likely to get and it keeps good steady temps for a few days, you should be fine. If you are using water, or even moist substrate to humidify the entire incubator there is no reason to seal the egg box. As long as the heater is powerful enough, it should get the job done for a single clutch..