[Ron Earp]

Well, every 300 mm on the tunnel you could cut a 1" diameter hole with a hole saw. Then, you could fabricate 2" circles from aluminum to cover them up, and, to give you an easy way to have an access panel for the afixing.

You could either rivet the line to the panel, then rivet the panel to the tunnel, or, rivet the line to the tunnel right next to the cutout and use the cutout to help get it riveted.