I built one out of 1" square tubing. I made the end open, and hinged it to the wall, and added a wheel to the bottom. I also added some pvc tubing (cut into 1" pieces) over pipes on the bottom, so that I can roll heavy material (think 1 3/8" MDO) in and out. It tucks up against the wall by the garage door (there is a 12" dead space in my shop).
After building it, I figured out the weak spot was the single wheel at the bottom (I kinda - sorta followed some home improvement plans that I've seen in some mag). Now I gotta go back and take everything out, and add some extra wheels at the bottom to make it more stable.