Styrene tunnel liner
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 smart thing to do is to hold it with garden stakes until you have buried it.
It seems strong enough. I am careful not to load it directly with too much heavy rock. We'll see how long the styrene lasts buried outdoors.
But the liner snapped twice trying to line the second tunnel - unless you do it on a hot day or apply heat, don't try bending 2mm sheet below a diameter of about 250mm.
Also the sheet seems to have a "grain" and bends more readily in one direction than the other.
The second tunnel (not shown) is in fact lined with five smaller overlapping panels after I ran out of big sheets.