The incubator I use and sell is a simple but effective.  It is fan assisted and keeps the temperature even in all areas of the incubator.  It has been well tested over 8 months with 12 successful bird egg hatches including poultry, quail and duck eggs.

It is made from a polystyrene box which has excellent insulation properties.  Temperature is controlled by an adjustable thermostat which is designed to keep the incubator conditions within 0.5 C of your desired setting.  The heat source is a low cost 2000 hour halogen bulb which creates enough heat to raise the temperature easily up to 40 C in an ambient room.  I use bulbs because they are very safe, easy to replace and inexpensive. 

The fan circulates the heat to ensure there are no hot or cold spots inside the incubator.  Humidity is controlled by adding water to a container supplied.   This raises relative humidity to around 40 -60% where hatching is possible.