It is a piece of tube similar to scaffold, galvanised inside and out. U-bolts have to be bigger though. The edge of the dish does flutter slightly in the wind, may have to stiffen it with extra steelwork at the back.
On another subject, Lady H wanted $ky, even though I said Rupert Murdoch has more money than is good for him, and more influence than is good for us. We have had it for 3-4 years, but she now wanted $ky+HD. The young "engineer" came today, I told him that I had already installed the cables properly and neatly, he had only to change the LNB from a 4 output one to a 6 output. He put the box in the house, and without checking the signal to see if it was OK, he swapped the LNB. No reading on his signal strength box. So he put the old LNB back. Still no good, so he swapped the dish, exactly the same. To cut a long story short, several hours later, another man came back, and tested the dish/LNB which I had aimed myself without an instrument. I was already tuned in, and he said the signal was perfect.
By coincidence, the young lad's previous job had also failed because the dish couldn't see the satellite! Moral: always have a spare signal strength tester.