Building a Camera ViewFinder in Flutter

Question

I want to build a viewfinder on top of a CameraPreview similar to the image below. So far I am Stacking a CameraPreview() with a Container() that has .5 opacity to darken the area outside of the viewfinder Container() that is on top.

My issue is twofold.

1. How do I cutout the top viewfinder Container() from the opacity Container() so that only the viewfinder Container() shows up bright?

2. How do I adjust the viewfinder Container() boarder so that it resembles the image below?

Code below image.

                 Stack(
                      children: [
                        Center(
                          child: CameraPreview(
                            controller,
                          ),
                        ),
                        Container(
                          color: Colors.black.withOpacity(0.5),
                        ),
                        Padding(
                          padding: const EdgeInsets.all(8.0),
                          child: Align(
                            alignment: Alignment.topCenter,
                            child: Column(
                              children: [
                                Container(
                                  key: containerKey,
                                  height: containerSize,
                                  width: containerSize,
                                  decoration: BoxDecoration(
                                    border: Border.all(
                                      width: 5,
                                      color: Colors.orange,
                                    ),
                                    borderRadius: BorderRadius.circular(10.0),
                                  ),
                                ),
                                  ],
                                ),
                              ],
                            ),
                          ),
                        ),
                      ),

'''