Since you have 3" of throat mounted on the device, it seems to me that the best thing for you to do would be to complete the horn by adding a flare. A 3" section has a quarter-wave mode at 1kHz and half-wave at 2kHz, so I think the best thing you could do would be to complete the flare by adding a horn as the device was intended. I would prefer to either have a direct radiator or a full sized horn than a truncated pipe.