My vote is that you check into building one. I think you will have little trouble covering the 100-600 range and can probably do it nicely with an easy-to-build horn having straight side walls for its flare.

Where will these be installed? As has been mentioned, that's very important and goes to the point of how large the horn must be. If it will be used indoors, and particularly if used in corners, the horn can be considerably smaller than if it will be used in freespace.

Wayne