I did one for a commission a while back.
I replaced the normal sponsons with Land Raider sponsons, trimmed down to fit the Pred. Has the added bonus of being easier to attach magnets, and is stronger than that ridiculous single peg that the Predator normally has.
Whacked a magnet in the middle of the sponson post, and on the inner side of the sponson weapon, with the inner sides of the weapons trimmed so that the sponson post sits in at the same point where the normal Predator post would have.
For the turret, I took off the curved piece on the weapon that normally fits into the turret, shaved the back of the weapon down flat, and added a magnet. Then added a flat plate inside the turret with another magnet.
I don't have any pics of the actual magnet set up, but the tank looks like this: