Don't put in stoppers. They can be used during incubation to aid in regulating but need to be taken out for hatching. Chicks need air. Most of us throw the stoppers away and keep air flow full entire hatch. If you need another water well or sponge in there to get the humidity up then so be it. The real deal is that many sponges should be enough, I suspect your hygrometer is off. Do a salt calibration on it.
