This can be a good way of mounting an aerial, either for the occasional greenlane outing or more permanent use.

On aluminium roofs, using a second magnet on the underside of the roof (if you have no headlining) under the aerials' magnet will make them attract each other, clamping the roof between them and holding on fine for flexible or small aerials.

As long as you choose a strong enough mount for the aerial you want (we can advise) it shouldn't even move, let alone fall off at motorway speeds or when hit by branches etc.