I put my two lights on the back wall as well...low enough not to melt the lid...but as high as I could. I have the fan at one end blowing across the lights to circulate the air. I do find it can be a cool spot just under and slightly in front of the fan. I'm replacing the fan I had with a computer fan with the hopes it won't be blowing as hard and it won't create a cold spot.
This is my set up. I tried to keep it simple for easy cleanup. That's the fan I'll be replacing. The floor is false with a pan I can add water to for humidity.
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I do wonder if my next incubator would be better if I had the fan in the center of the lid blowing upwards so the air would circle up, across the top and down the sides to be circulated upwards again.
I've also seen a cooler incubator that SallySunshine made where it's on it's end and it's equipped with trays for the eggs that can tilt.