We will start by flattening the box and dividing it into 2 equal parts with a curvy line right in the middle. Cut the box into two (we can get 2 file holders from one cereal box).
Cover one side (or both) of the cut box with black paper.
Draw your piano keyboard on white paper, cut and stick it to the bottom of the holder (I suck at drawing so I made mine on a drawing program and had them printed out). Cover the whole bottom part with the keyboard.
Repeat the same steps for the other half of the box.