Aaron and I dove into this for a day at his place this last summer, and the best we could come up with was Al's kit.
Set the barrel up to be about the size of a duck call barrel, 3ish inches, 5/8" bore. Then on the insert, make it similar sized to a duck call and neck it down on the end cap with the rubber bushing.
Your voice and your hand work play a huge part in the whole deal, and it takes a ton of air, and a ton of back pressure.
I still have not created an owl call that I would hand to any regular customer thinking they would do good with it themselves. I am confident this design will help call turkeys. A slamming car door will work to do that too though. I want something that is easy to use, and sounds like an owl.
Check out a Palmer Hoot Tube for ideas and dimensions if you want to make a fancier call with a little more hollow sound and a little less hand work (though a lot of voice and a lot of air will still be needed).
Vince