A fan or fans to draw the air across the heat tape contained in a false wall. Ideally a fan located at the top of the false wall to push air in and across the heat tape and another to draw it out of the bottom and recirculate through the incubator. These are critical since they create further buffering of the changing temps within the incubator. A single or few air holes in that container are all that is needed to provide air to the developing eggs. Accurate temperature monitoring.
Digital thermometers are cheap and ideal, the use of two is wise to ensure true readings.
Locate temperature probes in the egg chamber for critical temps. All air surrounding this container will fluctuate a fair bit but the temperatures within the container stay remarkably constant with only slight deviation. A predictable air path. If you are incubating large numbers of eggs then create an air flow across each box with baffles. Ideally, create a zig zag pattern of air, this creates even temps and avoids the dreaded hot spots that can destroy some if not all of your eggs.