My companion actually made me one out of an wooden box that he built, not knowing for sure what he was doing. We use a light bulb to heat it and sometimes it works really good, very had to maintain the temp.
The next one he built me was using an old refridgerator. It is smaller than average fridge. We use light bulbs in it also, but have had trouble with maintaining regular heat, we have an electric therm. in it and it varies about 10 to 15 degrees, not good. We are going to try a waffer therm. and a fan and keep trying. I would like to have a nice big one so we are not giving up. Mean time I have one on my table now full of eggs, that we just bought.
I hope to go chicken crazy, (I'm slightly off the edge anyway). And hatch about 200 or more eggs a year between, chicken, ducks, turkeys, geeses, etc.
Good Luck