If you're adventurous, you may want to build an exponential horn, but a conical one may be good enough - it's up to you. The applet will allow you to design both styles. Well, not exactly. The improved efficiency of the horn is gained at the loss of bandwidth. So, it may be louder but it will drop off in frequency at the upper end before the tweeter takes over if you're not careful.
Maximum efficiency is achieved when they are the same size, but the bandwidth is also the smallest.
To increase the bandwidth, you must shrink the hole. Those graphs show what your transducer is capable of. You can also use this approach to evaluate the suitability of a transducer for horn-loading. They are fairly inefficient as well. Theres is a patented band-pass design that works very well.