Cheapest way to make one is by taking a styrofoam container and filling it about halfway with water. Put an aquarium heater inside to provide the correct temperature, and place a slightly ventilated rubbermaid container with substrate and eggs in the water. Put the styrofoam container lid back on and you're set. Make sure that the holes in the rubbermaid are not large enough for a little corn to crawl out though. Good luck with your project