Asw, if you use the 'noodle foam' you can do as Al says and just make a tube out of pvc to keep the top out of the water (be sure to cap or plug the pvc to prevent water from entering the line as the pool fills with rain, snow and ice), the foam will take care of 'ice expansion. The real trick is to keep the foam from from floating, it's got to be wedged in so that ice will crush the foam and not crack the skimmer body.
If you go the 'Ultra-Gizzmo' route, pry the cap off before tefloning the threads and screwing it in.