Somehow, I was completely unaware of the existence of these exercise pens until this past Summer. Even when I saw them, I thought that they might be too light-weight to be of use. I couldn't have been more wrong! I bought the pen with a door last Summer to use for house-training our new puppy. The octagon above would have been too big for the space we had to put it in so I pried the hinges apart and made it smaller (6 pieces formed a rectangle-with the door between the 2 squares). 

It worked great--so well in fact, that I bought a 2nd pen that I move around as needed to keep my inquisitive and rambunctious dog out of areas that aren't safe for her. Set up in a zig-zag, these are really pretty strong and stretch pretty far. They come with long U-shaped anchors for more security outside, but I've never used them.