Your idea is close to what I have been thinking of for the window box. Bob has free roam so I am only trying to get it so he does not take off out of the window.
I used to trust just the screen but have learned better. He lost a toenail to a steel window screen when birds flew over head and only that toe was on the screen when he took off for cover. Now he is large enough that I bet he could push one right through the window and take off if he wanted.
I would not be concerned with the cement being toxic when setup. Think about it this way you use it for pipes carrying water you drink. I was thinking for convienience having the cage collapsable.
Yeah, the problem is that I didn't think far enough ahead to realise the mesh would help hold it together. The way I made it, all the sides unbolts so they can be moved, but they can't be broken down into the pipes.