For anyone else interested in designing a waveguide, there is a very interesting whitepaper by peavey. Although it is intended for compression drivers, their waveguide is much shorter than conventional horn lenses, and it appears the waveguide shape is very similar to the one linked previously with hifi speakers. Anyone wanting to build a waveguide will likely find their diagram showing how to work out the cross section very helpful:
http://www.peavey.com/media/pdf/aa/qwp1.pdf
note: pdf file, hence you may wish to right click and select "save link as"
http://www.peavey.com/media/pdf/aa/qwp1.pdf
note: pdf file, hence you may wish to right click and select "save link as"