This is what a very large number of commercially produced horns are. An easy to build flare is of course the conical flare. In fact, it is the easiest - not as efficient, but a lot easier to build. Just four angled bits of wood will do it. These sort of horns are sometimes seen as the mid-range part of a P.A. system.
Parabolic horns are so inefficient that they are really of no practical use.
So, there you have it. 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.