We have pet screen the entire height of our sidewalls, and standard screen on top. Our cats (20lb Maine Coons) will climb a lot higher than 18" if a gecko is running up the screen
We have a canvas covering over 3/4 of our cage - ~1,000 sq ft. The installer did not have to "beef up" our frame to accommodate the canvas. If an installer is talking about having to increase the integrity of the frame, I'm guessing they were designing for minimum load to begin with, which would have me concerned.