I've seen them. It's a piece of angle that has 1/4" rod welded into
the backside and bent into hook shapes. Basically, if you were to hold
the angle so it points upward (both legs down at 45 degrees), the
"hooks" would be welded inside the angle, starting straight down before
they curve into a 90 degree hook or bend. I don't know where to buy
them, but if you plan on having them outside, I'd go with stainless
steel if it's available.