I absolutely love this idea, we made a catio for our fur babies, and there is a small section that was grass / they have been using it for a potty area, and it needs to be upgraded and easier to clean. Do you clean the aquarium gravel or replace it with new?
Back in 2007 I turned one of the spare bedrooms into a catatorium and then built a catio just outside of that rooms window. I installed a cat door in the window to give them access. Here are a few pics I took back then.