Ok, you have me convinced. Sonotube is the way to go.

If both Giskard and Ian invented that angled cap trick at the same time (great minds think alike ) then I cannot pass on it.

Baron, I think I will not be able to cut that slot for this project: I want to install the dogbox in an existing cab (L200) so acces to the rear of the baffle will not be easy. But this is already on my todo list for any new project!