Mine was much much much easier because it was a pre CNC'd kit. Pre cut, pre drilled, etc. There is no way I would even attempt a tapped horn otherwise. Call me a wuss, I don't care.
Anyways, I am soooo not the guy to ask. There are already designs that people have shared with each other, and that's what I would do (because they already figured out the math for you, with specific driver parameters in mind).
If you needed a special one of a kind build to certain specs, then a lot more work, or a lot more help would be needed. Go hang out in the DIY forum here. Check out the stickies too. There are a number of people who will help you. Probably not me though, sorry!