I set one up like yours last spring. I did not cut the windows in the top and did not screw the wires down to the side. I figured I might want to use it as a cooler again someday. I also used just 1 ft of flexwatt and it was perfect - it created just enough heat. Even if I opened the cooler for a minute or so moving things around, it was back up to temp in a couple minutes.

Perfect hatch rate last year..

Thanks for posting the design!