I figured I would answer your questions here since some other people were inquiring as well. Here is a link to where the heating elements are listed:
http://www.lyonelectric.com/marshcat.pdf
Page 25.
The one I'm using is 250W. Your best bet is when you are ready to order give them a call and let em know the inside square footage of your incubator and the length the heating element will need to be stretched, they will get you the correct one from there.
You do not need to run the heating element to any sort of circuit board. All I did was run heavy gage insulated wire through the floor then attach the male section of a standard extension cord which plugs into the thermostat just like your heat tape or anything else. The heavy gage wire works well to prevent the heat coil from contracting back through the insulators. You can get an idea from the picture below.

Now with mine I did use a circuit board so I would be able to run the 12 Volt fans and the 120 Volt thermostat off the same power input. This however was on the other side of the thermostat from the heat element, it's between the wall outlet and the thermostat not between the heat element and thermostat. I attached a simple diagram of two different ways of doing it.
Hope this helps,
George Knaack
GLK HERP
