When I find your post here, it comes to me a solution. As all screens have differents proportion of Width and Height. Why not do it like this
1, use the Behavier Anchor, to fix the elements, whose positions are supposed to be fixed to the bord.
2, use a background a little bit bigger than the windows
3, Then chose Fullscreen scale.
As we know, the cocoon use a mixe mode of scale and crop, so it is a excellent solution:
We will have a reel Full screen mode in all device