I don’t know if I understand correctly because I don’t speak English.
You probably need to use a size, position and rotation filter. In your case, it’s best to add it to the path.
In the size field, enter the resolution of your input material 4096x2160. Now you will be able to reposition the image.
I suggest setting the resolution of the project probably 4096 × 1716. I don’t know what aspect ratio to enter for 2.39∶1 and whether in Shotcut it is possible to get letterbox, Cinemascope. If you manage to do this, you may not need a mask.