You first have to create a point list from what you have. The points could be the actor positions, but that is not very precise. You can work with bounding boxes there too.
So you have your BorderManager or something object. There you have a function. It calculates inside points, and with the algorithm then calculates the vertices of the boundary polygon. You can save them. Everyone can query these e.g. for drawing.
You don’t need to calculate it every tick, only recalculte when the user places a new building.