Water mover whats-it

My neighbor has a fair sized pond for their ducks and some cranes we have that live year round in the area. They are trying to figure out a way to keep it from freezing over if this is a bad winter. The last years they have gone out and opened it by hand but they can not physically do it this year. I told them about a coffee can and wood paddle wind thing my Dad made, but as we were talking we got to wondering if something made out of styrofoam (sp?) would not work better. Has anyone got any ideas? Or has anyone seen something they think would be "makeable"? They are like me, want to make it not buy it. LOL