What I would propose is a simple conical horn. The bandwidth you require is mostly midrange, so you might consider a higher crossover point which allows a smaller horn. Naturally, deeper extension will require a larger horn. If you plan to use subs underneath, you could raise your midrange horn crossover point an octave or so, which would make its size requirements much smaller. Then you could go with a straight horn, which is about as simple and pure as it gets.