Flutter – Positioning custom AppBar without SafeArea

I am trying to create a custom AppBar, with a polygon instead of the bar, I already have something:

As you can see, on some screens the bar is to high, because the SafeArea differs. I did not use:

child: Scaffold(
    appBar: MyCustomBar(),
    body: // my body
)

I wrote followings, which is a Stack where the bar is on top with a fixed height and a padding (SafeArea), and the main view takes the entire height with a padding on top:

child: Scaffold(
    body: Stack(
        alignment: Alignment.topCenter,
        children: [
            Flex(direction: Axis.vertical, children: [
            Flexible(
              child: SingleChildScrollView(
                      child: Padding(
                        padding: EdgeInsets.only(
                            left: padding,
                            right: padding,
                            top: padding + StatusBar.getHeight(),
                            bottom: padding),
                        child: Column(
                          children: children,
                        ),
                      ),
                    ),
            )
          ]),
          MyCustomBar(
            title,
            iconLeft: iconLeft,
            iconRight: iconRight,
          ),
        ]
    ),
)

where static const padding = 15.0; which is obviously the problem, it worked fine for most of the Android devices, but I needs to be dynamic based on the SafeArea of each device.

Why I decided to do it this way? Because I have a polygon as a bar and the text has to be visible where the bar is intended to be transparent:

The first solution I have in mind is to get the top padding of the SafeArea and using it instead my static variable. The problem is that I am not able to get the padding if I am not using the SafeArea.

If I refactor my code following this example in order to use the Scaffold.appBar, I get the following result:

IMHO I don’t know if it is possible to achieve it using the SafeArea.appBar.

Any hint to fix my MyCustomBar?

EIDT:
I just found this:

Scaffold.extendBodyBehindAppBar

This allows me to use the Scaffold.appBar.