The Meow't ® Cat Patio mounts in any slidey uppy-downy window 22 1/2" to 36" wide, has a flap door to minimize heating and cooling loss and a translucent roof so you can keep an eye on your cat-child. The kicker is the ventilated paw-print porch itself, of course. The porch section is 11 1/4" by 12" x 11 1/2" tall. At $150 from Improvements it's not the cheapest cat toy you can buy, but I'm thinking it just might be DIY-able! Via.