When you need to print your own horn for servo motors, the 3D printers produce slightly different results so that the horn may not fit to the servo.

With this tester a bar is printed which has holes with a radius of 2.7, 2.8, 2.9, 3, 3.1, 3.2 and 3.3 millimeters.

So print the tester out, attach it to your servo and find out the radius you need for your horns.