Since your plan is to use these with seven π-18's, I'd stick with the 90x40 aspect ratio. I think that's the best pattern for use with cornerhorns. I'd probably keep the mouth area the same, and simply extend the horn back to where it makes a 4" x 4" throat. That will look nice, making the midhorn width equal to the width of the bass bin. And I expect a horn of those dimensions will have very nice response too.