Like Jayde, I built my own. Base is a large breadboard for stability. Over that, a layer of aluminium foil to reflect any escaping heat back up. On top of the foil, a double layer of cork floor tiles for insulation, then a heat mat on top of that. Built an 8 inch high collar of cardboard around the heat mat and used duck tape to secure the lot to the breadboard. Half inch layer of vermiculite on top of the heat mat to allow air circulation and prevent overheating. Put the egg containers on top of the vermiculite layer with a thermostat in one and packed vermiculite around the sides of the containers for insulation. Layer of 5 old towels over the top of the incubator for insulation which allows easy access to check eggs. All a bit "Blue Peter" (UK people will understand that one !), but it's rock-steady at 82 degrees, 80-90 percent humidity and can be taken apart and the componants re-used/thrown away at the end of the season.
Cheers
Kel