The Frame: Use pressure-treated lumber for outdoor durability, but ensure it is dried and rated safe. Cedar is a naturally rot-resistant (and beautiful) alternative.
The Mesh: Never use standard window screening—claws will shred it in seconds. Use 19-gauge galvanized hardware cloth (1/2" or 1" grid). It is "escape-proof" and "predator-proof". Check out these highly-rated mesh options on Amazon.
The Flooring: If the catio is on grass, use stepping stones or cedar planks to avoid muddy paws and fleas.