I have successfully tree knocked with manufactured handles (i personally recomment a pick axe handle made of hickory - I have 100s of knows on my ine and it is still in fine shape)
As for finding wood and then knocking in nature.
I will go back to an expedition I was on in Michigan's UP...If you find standing dry dead wood, typically with signs of splits from being dry with no bark on them...these do not shatter or break easily. And they make good sounding knocks.
It takes time to find them however.
Also, a stick or log depending on definition of 6-10" diameter works well. I have had success with that as improvised knocking sticks.
lastly, I was shown sounding blocks. two 3" round cylinders of wood about a foot long. Strike them together makes for a higher pitched and resonate sound. I have tried to replicate this in nature with limited success. Again in finding a dry, old, barkless branch or tree to fashion into knocking sticks.
I think the points of Clapping and Vocalizations to sound like wood knocks make great sense. I would also put forth chest slaps or chest pounding as well.
With this in mind though, what is the audible range then?