Here is a kit that lets you add a small cat patio to your window. The Cat Solarium Catio gives your indoor kitties the opportunity to bask in the sun. It comes with a solarium, plexiglass back panel with a cat door, cat perch, pillow, mesh screen, and plexiglass weather enclosure.
The Cat Solarium is handmade in Alabama. It is water and sun resistant and is strong enough to hold 50+ lb load. It fits in most common windows. You can find it on Etsy.