The singular task of finding just the right HF horn for this project has taken at least 4 times longer than it took to design the midrange horn! A pair has to fit exactly across the mouth in the vertical, they can't be too wide or too panel-like, they must be constant directivity, 90 degrees, and have a cutoff between 1.5kHz and 2kHz. I've even considered using a bi-radial horn sideways with a deflector to correct for upper-range dispersion!