Actually, the "still air" model is preferable. The fan model will contribute to drying of the eggs, and will make maintenance of lower temps more difficult (the fan motor generates a lot of heat). A fan is required only within very large incubators to prevent vertical "layering" of air.
HOWEVER.... an incubator is simply NOT required for corn snake eggs. I've hatched out several clutches (100% hatch rate by the way) by keeping them on a top shelf within a closet. Anywhere from mid to high 70's to low to mid 80's is ideal, and fluctuation within this range is acceptable.