Cap'n Crunch cereal box
Cap'n Crunch Whistle was a cereal box toy that phone phreakers were able to use in their shenanigans. If one of the holes was blocked then it made a 2600 hertz tone that worked very similar to a blue box. John Draper was nick named Captain Crunch for making the whistle famous, but it has been reported that he learned it from Josef Engressia aka Joybubbles.
It can be 3D printed from Thingiverse.