Incubator?

I have a gravid female BP, at least I think she is gravid. Her and her boyfrind locked on 11/14/08. Since then the females colors have brightened up a lot but she doesn't look any fatter. This is my first time breeding ball pythons. I was wondering if any of you could tell me how to make an incubator or what works best for you. I have seen a few online made from styrofoam coolers and such. Do those work?

Thanks!
Jenny

heres a link to a diy fridge incubator. and as long as it holds the temperature you want it will work. http://www.arbreptiles.com/cages/incubator.shtml