I see all the great home made incubators on here. They find all kinds of thermostats, heat elements an insulated boxes. But they all have the same flaw that the commercial models have. Big $1000 models all the way down to the $50 foam ones all rely on air space as thermal mass. They assume that the more airspace in the incubator the more stable the temperature will be. Which is true if your thermostat never acts up an you never open the incubator.
Air is about the worst thermal mass you could use. Air changes temperature very rapidly an if your thermostat acts up it will allow your incubator to make drastic temperature swings in a very short time. Also, the nature of heated air makes it rise rapidly so any time you open the incubator the heat literally jumps out of the incubator replacing it with cold air in less than 3 seconds. So no matter how fast you turn your eggs you still lose all your heat. Then the air starts pulling the heat out of the eggs to bring the incubator up to temp. So the eggs cool about the same amount if you open the incubator 3 seconds or 3 minutes.
I think that every bit of free space short of killing circulation should be filled with more stable thermal mass. Bricks an rock are the best but closed jugs an jars of water work great. In the little foam incubators I fill the bottom with gravel an lay rocks anywhere I have space. The bigger coolers you can put blocks or milk jugs in the bottom an place a wire floor on that. The idea it to have as little empty space as you can in the biggest thermal box you can.
If you open an incubator built this way you loose less hot air an then the rocks heat the air back up quicker resulting in less cooling of eggs. You still wont to get in an out fast but now you are getting rewarded for it.
If your thermostat spikes to 107* then it will take twice to 3 times as long to get there giving you more time to notice an correct the problem.
If the power goes out it will stay warmer longer but on the back side, if it does get cold then it will take longer to warm back up.
You can also ventilate more because the air is no longer thermal mass.
The only down side I've found is that it take for ever to warm up from a cold start.
My latest incubator is a side by side fridge out in the weather. It has a cheep mercury bubble thermostat off my wall (the cheep central air kind) Turning on an off an outlet with a hair dryer plugged in to it. The fridge has a shelf with a turner out of a foam incubator(more to come). The freezer has the hair drier, thermostat an is full of 8 inch blocks. (There is a fan circulating between the two.) And both doors are full of jugs of water.
Ive been testing for close to a week an it keeps temp(air, no water weasel/wiggler) + or - 1*. Yesterday it was out in 85* an last night it was 18* without any change of temp in the incubator. I even opened it about 3 AM for 30 seconds an it to less than 2 1/2 minutes to be back to 99*
North West Georgia