Since the other cooler incubator thread turning into a mildly amusing drama, I figured I'd post mine. I built this incubator back in early 2005 based of some plans on the internet and some general common sense.
I have incubated multiple ball python and green tree python clutches in this since. (even both at the same time)
While this design is more money, I think it is far more practial than a styrofoam one in durability alone, not to mention size. I have had this "tutorial", if you will, posted on my site since it was built.
Cost was around $200, but half of that is the thermostat and the cooler was over $50.
Enjoy.
http://www.degei.com/supportfiles/howto/inc/index.htm
And just for fun, here's what it's doing right now. (due the 10th)
Oh, and I should add, my "egg boxes" are a Marth Stuart 9x13 casserole travel thingy I got a Kmart.