This image shows the contemporary border between the Dutch Republic and the Spanish netherlands. A border which has been established at the end of the Dutch war of independence due to failed attempts of the Dutch to incorporate Flanders into the union. Resulting in the Netherlands receiving a small parts of Flanders only, dubbed 'Staats-Vlaanderen'
. This border is noticeably historical inaccurate considering these events took place several centuries later after the events of the Hundred Years' war took place.
Instead, the correct border should show it like this with in red the county of Flanders and the bold line representing the divide between the Kingdom of France and the Holy Roman Empire. :
The upper northern part next to the Westerschelde is called 'Land van de Vier Ambachten'
or 'Vier Ambachten' in short. And it's not to be confused with 'Staats-Vlaanderen'.
Vier Ambachten was an administrative and juridical entity within Flanders which enjoyed great autonomy and had the statute of 'Keure' to the Holy Roman emperor. I'd highly reccommend to make this a territory in its own right and add the remainder part of Flanders subjected to the Holy Roman Emperor along with it. Please Apollo. Flanders was the wealthiest duchy in the french kingdom. This is a good excuse to add extra land