I have made and successfully used a "coolerbator". I heated mine with an under tank heater taped to a pyres baking dish that I filled with cypress bark, adding water to regulate humidity, it was a bit tedious to regulate without a thermostat but it worked well. I've also made quite a few enclosures for reptiles. if you really want to stick with a light bulb for heat, I'd go back to the 40 W bulb and add more insulation to the outside of the cooler and use a can of spray foam to glue it all together and seal it up. for ventilation, I'd get a little plastic tubing and drill appropriate sized holes in the cooler, large enough to snugly push the plastic tubes through. I'd place a few along the upper edge and a few lower on the walls of the cooler, the heated air in the cooler will rise through the top holes and draw in some fresh air in through the lower ones. the control humidity and heat, you can plug up the tubing as needed with tissue. you want to get the temp pretty much exactly 99.5. ideally you'd use a temp gun to check the surface temp of the eggs. a difference of a degree or two can really throw off the development of the embryo's. I use 3, different thermometers till I find one or two I can trust. you can use a heat mat instead of a light bulb, it has a lower fire danger. with a heat pad you can use a jump start thermostat, which can be had for under $30 and it will dramatically reduce your stress. best to place the incubator in a room that has a steady temp, a basement room can be good as long as you can insulate enough to get the temp nailed.
