The original sponsons are not that hard to magnetize. Do you just want to magnetize the weapons or the whole sponsons?
The weapon is easy to do. Just use a 3mm magnet and glue it on the sponson and the other inside the weapon hull fixed with
GS.
I have also magnetized the whole sponson. I just drilled another 3mm magnet into the inner door of the rhino chasis. The other magnet i drilled into a piece of sprue that i filed until the door/sponson could be glued to the sprue piece.
Just cut a sprue piece that is slightly to long, put the magnet in and the gently file it back until the door sits flush. If done carfully you don't need to think about alignment as you can attach the sprue piece with the magnet to the magnet already in the inner rhino door. Apply glue to the sprue piece and press the door/sponson onto it so that the glue bonds to it.
But you might want to leave a tiny gap on the doors (but not the sponsons) so that they can slightly move or you'll have a hard time getting them off the model.
If you want fully magnetized predator sponsons you'd need 4 magnets for all weapons 4 for both sponsons, 2 for the Rhino doors and 2 for the inner door.