Mark, Here are a few shots of different planes.
1st photo Small Futura On this one I just safety wired the smoke tubes to the pipe mounts at the bell mouth.
2nd Photo Comp ARF Tutor. As you can see in this one The smoke injectors are not run between the walls of the pipe just down the Fuse sides and out the back, This also works well.
3rd and 4th photo Large Futura. We just used Safety Wire in 2 spots around the inside pipe.

When we run the 4mm stainless tubing between the walls of the pipe we cut a small notch in the inner pipe at the rear so the stainless smoke tubing has a lock to keep it in position, we then pull if forward into the notch.
Hope this helps.