Yup, cooler incubator here. Use flexwatt, if you don't want to wire it up, Reptile Basics can do it for you. I have two strips of 4" flexwatt in the bottom of a big cooler. On top is a layer of plastic water bottles with water (temp stabilizer), and a piece of plastic light grid on top of that.

Works perfectly. Make sure you run a high quality thermostat though (Herpstat, etc). You want steady temps.