As the title says, anyone know how to magnetise the main cannon of a fire prism? Its huge, metal and I have no idea what to do with it

But I would like it to be interchangeable into a falcon. I've had a look around, but can't seem to find any good answers, besides a template for making a plasticard version of the prism (which isn't preferable).
I'm guessing that someone has done it before, so any ideas/experiences would be greatly appreciated. Preferably with pictures
Oh and no I don't want to get the new all-plastic fire prisms, I don't really like how
GW has designed them