Collect some unused Lego pieces. Standard ones have a 5mm centre hole. In Australia, neodymium magnets can be bought from AMF Magnetics. www.magnet.com.au Get the SupaMAG 10 pack 5mm x 4mm.
Put single magnets onto the reverse side of the metal to hold in place. Push the Lego onto the magnet to lodge it inside the hole. The 5mm width of the hole will form a tight hold onto the 5mm magnet.
Enjoy and get creative thinking of other uses for these Lego fridge magnets 😁