Because finding a perfect map for this project is kind of difficult, I will shape my own map! For this I need a vast plain and a lot of water on a high ground not too far. Finally, this map from @Boulderboy will be perfect, but instead of using this nice lake, I will drain it to the lower plain!
Here you can see the reservation work for the central island where farms and cattle will be:
So the next step is to clean the whole area and carve the big lake on 3 layers of soil only to get is the biggest possible.