Hi mate - Without knowing how the box opens for use it's tricky to give accurate advice. Does the top edge have a void i.e. no actual panel? If so then it is tricky to make out of cardboard. Is the chamfered triangle corner essential? You could vacuum form it out of plastic quite easily, just make a wooden template first but you do need someone with a vacuum former obviously.
If all sides of the box are solid, then the triangle would be best hard attached to the top side of the box with two tabs on the other two edges. Cut out the exact angle from the two vertical panels. If this IS the way you are doing it, then carefully open up a cereal box along it's tabbed join and you will see how to attach each side into a net.
A bit more input is needed from you before complete advice can be given!