[Ron Earp]

How is the inspector going to determine that they are fastened every 300mm?

You could use P clips and glue them on the lines every 300mm in the proper orientation. Then, you could drill holes every 300 mm. And, probably with a lot of cussing you could manouver the lines and P clips to correspond with the hole and get a pop rivet through it from outside the tunnel. Use of slightly oversized holes would help here I'd imagine.