You might be interested in the midhorn I did with a 10" driver. It covers nearly the range you're wanting, shifted about an octave up on each side. But if that will work for you, it will get you going right away. It's been tested and its performance is documented so you can know what to expect from it.Or maybe scale it up for a larger driver. I'm fairly confident it would work pretty well, at least put you in the ballpark. Maybe you would enjoy making one and giving it a listen.