12V PSU : mechanical Thermostat: 12V lightbulb. and return. Job done.
Look at these, especially the first one. If not one of these then google "diy variable power supply".
If it's a one off the airing cupboard will work - Keep an eye on them.
How many eggs. I built an incubator for 3 eggs in a small wooden box and a 25 watt bulb. I propped the lid open differing amounts until I got the temp right and about 30 days later I had 3 chicks. I guess you could do the same with several 12 volt light bulbs and a variable power supply.
Thank you for your answer. I am looking for a circuit to build a variable power supply for a chicken incubator to hatch two dozen eggs.