The Portable Playpen Pet Pen By the Armarkat Shop
This is a good investment.  My adult 15lb cat has enjoyed it since getting it.  I have an indoor cat, so I leave the top open for him, put some of his toys and there and he has his own area away from visiting children.  It does set up easily, and just as quickly can be folded flat.  I like that both bottom and top sides have a zip.  It is lightweight.  My pet loves to destroy things so I will monitor to see if it tears easily (usually the mesh vents go first with him).  I can't wait til summer, he wants to go out so bad, now he can come outside with me (doesn't like a leash).  It is bigger than I thought it would be, it would hold a small to medium dog.  It folds like the pop-out laundry baskets. Since, he is using it quite often, I have not put it back into the carrying bag which feels and looks strong. I would recommend this product.