Doug, are you talkinga bout a bite type, like these black plastic ones are, or are you talking about a regular ol' open reed predator call like a howler would be?
On the bite kind, I make my barrels 3" long, and I REALLY flare them out. The toneboard setup (I'm sure Al has some) is set up for a MT2 taper. So, typically, that would mean a 5/8" hole, and some kind of modification to either the guts or the barrel at this point for a good fit.
A good fit on those guts is VERY important to get a good, smooth sound.
I also like covering the bite part in latex tubing. It makes for a VERY smooth sound. You can use shrink tubing too.
If you're talking open reed like a howler, I've got a Primos I was running today that's part of a new bowhunter education kit I picked up today. Its a pretty good call, and would obviously work as a predator caller too. I can tell you I'm putting VERY little of the toneboard in my mouth to get a cow elk.
Lots of rambling there......let me know what you've got in mind, and I'll help out more with some specifics.
Parker