I am a flat cat advocate too, we had one for the bedroom window when Denzel and Dexter were kittens so we could have the window open at night without the fear of the cats getting out.
There is a flat roof below it.
I rang them re the size as our window wasnt a standard size, and they just sent a larger one rather than them having to make one.
we dont use it any more, but they are sturdy, robust and goor value for money!