You can actually replicate it to some extent. It's what is called a radial magnetic field, meaning the magnetic field lines extend from the center to the edge. Apparently you can make permanent disc magnets like this too. Of course, there's never been any explanation as to why the Flat Earth would have such a magnetic field. And I haven't seen any evidence that the Earth's magnetic field actually conforms to what you'd expect from this kind of magnet. It's the same old thing really - we don't know exactly what it is, other than it perfectly matches what you'd expect on a Round Earth, while not actually being caused by anything to do with the Earth being round.