MIKEDE 12 Pack Strong Neodymium Magnets, 20lbs Black Countersunk Magnets for Wall Mounting, Heavy Duty Round Base Cup Magnet with Stainless Screws for Kitchen, Tool Room, Industrial - 0.63 x 0.19inch

You can buy magnets with holes for screws in the middle, often with steel backers that make them even stronger. Cut a shallow flat-bottomed hole into the foam gun, fill with plastic-weld 2-part epoxy, sink the magnet into it, and dab more epoxy into the middle of the ring magnet, such that the epoxy goes through the screw hole, replacing where the screw would go. I bet the foam tears out before the epoxy lets go. And a magnet like that (or two or three) will be plenty strong. https://a.co/d/0eTRnPRS https://a.co/d/070MHqIg