I just wrote an article in the french forum about the CBGs and other instruments that soldiers made during the"trench war".
Even if I found more documentation about french parts it seems that this kind of realization was made by all countries soldiers involved in this war. You can find easily of Canadian or German soldiers (see last images below) which such instruments made with wood but also helmet or gaz mask!.
Here come some images and a link:
Link in french with more pictures