I made several.
One I used was a wine fridge that I got on craigslist for $30, packed the sides with insulation I got from a mouse order, hooked up a 20 gallon UTH to a thermostat and placed that in the bottom, and had a digital probe thermometer in with the eggs.
The second (and third) are giant styrofoam shipping boxes that I got from work. I use a 20 gallon UTH, and the thermostat again. I measured about 3 inches up two opposite sides and marked every 2 inches or so. Then I bought some 1/4 dowel rods, cut them to length, and poked them through the box to make a platform. The UTH is on the bottom and the egg boxes rest on the platform and can stack on each other.