Like portable air conditioners, these window extensions fit into double-hung window frames and are generally constructed of thick, sturdy polyethylene and/or clear acrylic. Some models have accordion side panels that expand to fit windows of various sizes, offering a snug, bug-resistant fit.
The clear viewing panels offer a three- or four-sided view and gives kitty the feeling of being outdoors. Place a bird feeder nearby for extra entertainment. The floor is carpeted or made of opaque acrylic, and most offer venting that allows kitty to enjoy the fresh air and outdoor odors.
Advantages are price ($80 to $130) and ease of installation. The main disadvantage is the small size and confining nature; these don't provide exercise opportunities. Also, they can only accommodate one cat at a time, so be sure to buy one that's safety tested, weather proof, and guaranteed to support the weight of at least two adult cats.