That sounds pretty frustrating. If the grid snapping breaks down that far from the center, what about things like movement speed and projectile collision? Even if you could find a way around the grid snapping, you might still run into other problems if the engine loses location precision far from the origin.
Maybe you can assemble the thing you want to make near the origin, then move everything together out to the edge of the map. Or you could assemble it, then make a prefab, and drop a prefab where you want it.