As Wayne mentioned the throat in your design is extremely small. I think 100cm2 or more would be more realistic. I think you will see that the upper cuttoff frequency of the horn will be come less. If this is the case don't worry to much. Hornsresp only gives you the SPL produced by the horn. The speaker itself will have its own sound.In practice you will need a volume in front of the speaker to couple the speaker to the horn. For ATC you should enter the cone area and the volume should be at least cone area X maximum excursion, this to prevent the cone from hitting the panel which is in front of it. Try to keep this volume as small as possible because it acts as a low pass filter which attenuates the higher frequencies.
Best regards,
Walt