I have to admit that my incubator is a bit small. I can hold 15 eggs, that's it. BUT I do think it's more forgiving in the heating department as it gets back to temp pretty quickly.
I use an
electric waterheater lower thermostat. I drilled holes in the back (into the ceramic black backing) It's hard as heck, but just keep drilling away and you will get through it. I hang the thermostat with the main central hole (on the side not shown in link) right next to the light bulb and try out different settings until the 'bator holds a good range. This is because, the light bulb will quickly heat the thermostat, and when it's off, the difference in temperature is greater than if the thermostat sits farther away so it will turn back on again faster. Before I hung it close to the lightbulb, I got temperature varience of 10 degrees! WAY off. Now it's a matter of 0.2 degrees. I think I have it set just above 125 degrees mark. I use a 40 watt mini bulb (the small round ones you see in bathrooms or theater mirrors) and a computer fan set at 5 volts (I was worried it wouldn't be enough air movement in my tiny setup , but it seems to be doing very well.) I used an old cel phone charger which is 5v to plug it in but you can use anything up to 12v. Now that I discovered that I need to have the thermostat right on the bulb, I don't think it was necessary to drill holes in the back for air movement through it. If I did it over again, I would at least try it out without the extra holes. However, the extra holes may be helping the thing cool off faster so that it switches the lights back on sooner?
I hope that was semi clear? It was really easy to set up and I did take pictures, but saw so many with photos of their setups, and mine was just another of the same idea, but I must say I didn't understand the wiring on most of the tutorials until it clicked (like a lightbulb over my head, LOL) So I don't know if it's because I'm dense, which I am, or if "normal" people would have just as hard a time understanding as I did. I may just make another tutorial 'cause I fancy myself as a good tutorial writer for the mentally challenged, ROFL
NOTE: I may have the temperature varience of .0.2 degrees, but the room temperature is always affecting the interior of my box, so I'm keeping an eye on it all the time, got to know what time of day it tends to get hot, and what time of night it tends to get cold (6PM and 5AM respectively) and I just plug my air holes or open them up to accomodate this. I'm constantly watching the bator, so it's definitely NOT automatic ;P Gotta stay near home for 3 weeks to do this ;P