I have been in contact with a very generous person named Bjorn Jansson ( certainly sounds like a speaker brand ) who has written software to calculate the cutting angles for making straight sided horns.
The shape - a pyramid with the top cut off is called a frustrum. One for your next round of Trivial pursuit.
The link to the calculator is
http://jansson.us/jcompound.html#recprism
I hope this can help some one - I am using it to build a set of 6Pi speakers
http://www.pispeakers.com/catalog/product_info.php/products_id/99
Regards to all