Hi, here's some ideas for Catios https://www.hometalk.com/search/posts?filter=catio
There’s a lot of them on-line, that you can copy.
Ventilation, separation from the litter box, a scratching post and toy area are essential. I would make it modular, so that it can be cleaned easily.
Dog kennels have a tray under the entire bottom.
See if these help you!
Here's a great post on building a cat enclosure.