I think the best figure for a cereal box would be a cylinder. There are no corners on a cylinder where it is hard to fit round pieces of cereal into. The formula for the surface area of the cylinder is 2*pi*r*h+2*pi*r^2. The formula for the volume of the cylinder is pi*r^2*h. (Then I would add some more info supporting why you think a cylinder would make a good box.)