They're some sort of cable boxing. I found them at work in one of our stores and the gaffer said they'd been there for about 10 years and no one knew what they were. That's when I thought "Hmm i wonder if the bend?" They're perfect, really strong (They have a curved edge and a double box sectioning). They gave me a headroom of 6'4" which is perfect for me at 6'. The rest is pallets and some fence posts I scrounged when the colony fence got trashed in the recent storms. So far I've spent about £6 on screws, nuts and bolts and eventually the cover will cost about £40.
I hear water pipe is the usual weapon of choice
I just got lucky finding this stuff at work.
ETA they're some sort of plastic.