After giving up on finding irrigation pipe at 300mm diameter and lengths of only 2m, I have used black 2mm styrene sheet as a tunnel liner on two tunnels.
The original timber lawn edging performed a retaining function at the top of a steepish slope so i didn't want to remove it but the "creekbed" ran under it, as we saw in a previous post.
Plastic trenching and sharp sand ballast make tracklaying go quickly and adjust easily. Soldering a jumper across every joint makes it go much more slowly!