My team used extruded aluminum the first 2 years, but it was way to heavy which made us look for an easier base material. The past 2 years we have used a producted made by cooper/b-line. It is designed like U channel or back to back U channel but stronger. It is very easy to use, and it has brackets that help with the initial frabrication before welding. It is also very light and strong. Here's the website, it describes it better than I can:
http://www.b-line.com/. I doubt that we will ever go back to extruded for the base. As for now, we'll keep that for telescoping arms.