Ten years ago at the SHOT show, I met the guy who started Flextone calls. Not custom, not wood, but he had a wood duck call that was a reed call inside of a flexible black rubber barrel and stopper. Being able to bend, partially close or totally close the exhaust, let him sound EXACTLY like wodies sitting on the water.
Again, this does not have to do with building a neat woody call from a shotgun shell, but if you really hunt wood ducks, that call is hands down the best sounding call I've ever heard. It seems to me that the squeal rarely moves the birds around, but if you can make the crisp sounds they make on the water, you can get woodies to change their flight path.
No attempt to hijack, and I only make wood calls, but just wanted to mention it.