To make an all wood grunter, just change the stopper end.

You still have to have it join to the barrel, but you turn the stopper end to replace the tube. There are a ton of examples here and on Facebook for ideas. It's really not that involved or difficult. If you are looking for EXACT dimensions of the exhaust tube, I don't believe I have ever seen that information posted here. Might have been, but I just do not recall it. It's something that you have to tinker with on your own to get your own sound out of the call.
Al @ THO