Well, you like the WO or WO32?  I would try to unfold it into the general shape you are going for, that way you are using a horn design you know and like.

Other than that, you would need to design the horn from the ground up, there are general shapes, like exponential, linear, etc. expansion.  Then the ration of horn throat to mouth and many, many, many other things to optimize the horn for your driver.

If you have the skillz, go for it!  I do not.