http://www.volvotreter.de/downloads/...le_Horns_2.pdf
If this helps, if you have a Basic Diaphram for an air horn"hence you can reuse old diaphrams(with X, Fe(frequency resonance)) you can look at the length x diameter x flare rate, to make the appropriate Horn loading for best efficiency.
Found on Page 5, Tables 4, 6, 10 will be the most useful info for your application.
While this is specific to Speaker design, Sound is universal regardless of diaphram drive type used.