DIY ?uestion

Well, it it tapers like a horn, it's a horn, not a transmission line. The big problem with transmission lines is there's not a terribly well defined theory on doing them. Typically the idea is to get kinda close, then stuff the hell out of them with fiber of some sort. Loading the driver with a horn at least gives you a better defined theory to work with. 

Transmission lines can be very satisfying... As koss_amojan explained in the past there has been a great deal of guess work... Maybe 20 years or so ago the math was fairly well proven based on figuring line length based on the cone size and what it takes to get a quarter wavelength of acoustic standing wave inside of the transmission line. It isn't typically for the beginner.  If you insist on a transmission line, I would highly recommend that you consider finding a proven kit and starting there. 
If you'd like to create an all new design of a transmission horn or something, that would be great,  we are always arguing within the forum  that there isn't real growth in speaker technology, maybe you can give us all something to talk about. 

That's the kind of info I need ....to determine that I need more info.  Will delay my build until I can get a better grasp of this.  Thanks muchly!!!!

Take a look at some of the old IMF's (U.K.) and how they built their trans lines. TSL - 80's. I'd just copy them!