This is a pretty clear case of someone buying gear without first considering if it was appropriate for the situation. The choices are pretty much to make the best of what you have or get something else that would be a better solution.
By the way, as Ivan noted, you may want to look a bit closer at the Faital horn you linked. Based on the information shown they may be about 55x55 at 4kHz, but at 1kHz they are almost 120x120, at 2kHz around 70x75, at 8kHz about 45x45 and at 16kHz around 30x30. And that is just the horn pattern without considering the effects of the enclosure, the interaction with the LF driver, etc. So what would be the resulting effective pattern for your use?