About 12 years ago, I found a private manufacturer of a window air-conditioner style unit with screens for the walls, and a solid plexi-glass top and bottom. The access side had a door that allowed you to open it or close it (and lock it), so that you could control your cat's access. It was made of aluminum/metal, was completely self-contained, and was very sturdy. Additionally, I was able to install it myself. Unfortunately, I donated it to a Massachusetts animal shelter. I've been trying for the last 4 years to find it again. I believe I located the manufacturer in a Cat Fancy magazine. I'm sorry it's not a lot of help as far as where to get one, but there is hope because it did exist.