I designed it, Ron fabricated it and my 11-year-old nephew Henry (and 7-year-old Oliver) helped to install it. It rocks (and it's filled with rocks... well, sand and decomposed granite, specifically).
It's made of mild steel and will hopefully take on a Richard Serra-esque patina over the years.
Here's a link to a higher-res PDF should you want to make one yourself. If you make one, drop me a line and let me know how it went.