Summary

If you connect one of these pure piezo speaker disc to your arduino tone() output, it is not very loud. Take one of these horn housings and you will have more noise.

This thing is designed for a 20mm diameter piezo. Scale in your slicer software to your required diameter. Inside there is a small square at bottom. You may use for a small circuit board. If your piezo has cables, make connection to the outer cables there. If your piezo has no cables, wind springs with copper wire and solder to this circuit bord for connection. Maybe you like very small circuits, you can place your beep circuit here.
Apply different frequencies and search for the frequency of the loudest noise. This is the resonance frequency of the whole horn, it differs from the resonance of the pure disc.
When your tests are complete, glue the two parts together.
Examples are made in PLA (blue) and ABS (red, yellow, acetone vaporized) , no support needed.