QRubberBand widget background transparent

[bmn]

Hello,

I am trying to get rid of the half-transparent fill of the QRubberBand widget. I just want it transparent.
What I already tried:

QPalette p = _rubberBand->palette();
p.setColor(QPalette::Background, Qt::transparent);
_rubberBand->setPalette(p);

QPalette p = _rubberBand->palette();
p.setColor(QPalette::Background, Qt::transparent);
_rubberBand->setPalette(p);

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and

_rubberBand->setStyleSheet("background-color: rgba(0,0,0,0)");

_rubberBand->setStyleSheet("background-color: rgba(0,0,0,0)");

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and

_rubberBand->setPalette(Qt::transparent);
_rubberBand->setAttribute(Qt::WA_NoSystemBackground);
_rubberBand->setAttribute(Qt::WA_TranslucentBackground);
_rubberBand->setAttribute(Qt::WA_TransparentForMouseEvents);

_rubberBand->setPalette(Qt::transparent);
_rubberBand->setAttribute(Qt::WA_NoSystemBackground);
_rubberBand->setAttribute(Qt::WA_TranslucentBackground);
_rubberBand->setAttribute(Qt::WA_TransparentForMouseEvents);

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but there is no effect at all. The rectangle is always rendered the same way (see attachment)...

How can I fix this?

Thank you in advance.