Your pour concrete, cement is an ingredient of concrete. I know it's not really the point of this post, but it's one of those things that really bugs me.

I would just install the window inside the frame, and them trim round with some PVC corner trim, which I think you can get in up to 60mm, so it would easily cover the missing parts of the siding.

Let it get a nice natural patina, then clear coat it, that way you will end up with a rustic look that will also last.