Rob,
Welcome to the Forum. Thanks for the steel info - I'm going to visit a supplier's drop shelves today or tomorrow.
Do you already have the C channel? If it is what you have, go with it. However, in my experience, it is not as stiff as box steel - it can twist - though 7" is pretty stout and very heavy! I made mine from 2x4x¼. If I were to do it again, I'd only go with 3/16" wall.