When I took a welding class at The Crucible, my first project was to design and build a cat-landing, at the same elevation as my raised floor diaphragm, that would allow me to install a cat door directly into the exterior wall of my house.
The cat patio (catio) framing is made of 1.5″ L-channel steel, with extra-long legs that are sunk into the earth about 8 inches. The L-channel makes a natural ledge for a detachable wood platform, which I made of yellow cedar to match my hot tub. I popped in some rubber washers to allow air circulation so the wood doesn’t rot. The roof panel is corrugated steel, easy to cut with tin snips, and it’s attached to the frame with all-purpose adhesive.
My cat took to it right away. She spends hours there, overlooking the yard, watching the birds, ready to run inside if that scary feral cat with the thick neck comes around.