I never achieved a uniformed air flow with a top mount heater/fan over a single layer of eggs . You may have better luck than I did. I hate to think of the time and money spent trying different designs only to discard them. I found directing the air front to back thourgh the egg trays gave me the best temp distribution. Keep in mind air flow also cools the eggs later in the incubation period. Developed embryos are exothermic and egg shell temp exceeds the incubators set temperature. I had real problems with my early hatchers as I insulated the cabinets and experienced runaway temps with energetic breeds. Cornish cross broiler chicken eggs were the worst. It causes you to vent more to off set temp increase that lowers the humidity. If I was in a cooler climate or had climate controlled room it many not have been a problem.I wonder if there is a way to improve the airflow with them mounted to the top? A single layer hatcher would’ve been the easiest to build but if I’m going to have hot and cold spots that won’t matter.