OK, here is what I came up with, and so far it's working just fine.
We are near Calgary, Alberta, and this week it's -28C (approx -20 F). everything freezes, except my eggs (...and my hen's eggs, which we are actually discussing).
I have uninsulated barn that has the same temperature as outside, minus the wind. I purchased heating tape from Home Depot - the kind you wrap around your pipes so they do not freese. I stretched it flat, drilled holes on the dividing walls between the boxes at the level of the bottom board, and pulled it through from the first to the last box and back (kind of an U shape when looked at from above). The tape has built in thermo unit so it kicks in when cold and turns off when warmer, and keeps the temperature at 5-7C ( 42F), just enough to keep the eggs from freezing without cooking them. The hens do not care for the wire that is under them (and is covered with straw). At the same time the tape is not warm enough so the hens do not lay in the boxes overnight...)
Today we had -28C and after coming back from work - zero frozen eggs. I am veryy happy with my "invention".
hope this helps you all.
link to the product:
http://www.homedepot.ca/product/15-ft-electric-pipe-heating-cable/909328