If I made an ice sculpture for the fair. And let imagen that I poses such a skill even. I would choose something to represent Hogwarts in itself. Such as the crests of all the houses. Perhaps an elaborate sculpture of 3 dimensional animals that represent the four houses standing over each other on a mountain cliff or looking up at the sky in proud confidence next to each other. Perhaps even make a small Hogwarts sigil at the foot of all these mighty looking creatures to represent our proud school and home.