I'm working on a tractor with the pen made of stock panel -- also known as cattle panel or hog panel. The panels are rigid enough that you can hook them together with cable clamps and they'll stand up on their own. You have to cover them with weld wire or hardware cloth, because the spaces are pretty big, but they're sturdy enough to keep out dogs, and light enough to drag without falling apart.
They come in 16 foot lengths, three feet or four feet high. About $20 or $30 per panel, depending on where you get them.
sorry I don't have pics yet.