I have a magnetic retreaval tool in my work trailer, and it is very handy. Once, several years ago, building management was taking me to a phone equipment room in a multi-story building when he dropped his key ring (with MASTER key) in the opening between the elevator and the wall. With no master key to access the elevator access, he was SOL. I taped the magnetic retriever to a fish tape and recovered the keys.
My magnetic retriever has a "keeper" on the magnetic end that isd basically just a piece of steel. The U shaped magnets we had in school also had those. They seem to significantly reduce the magnetism when they are on the magnet, and I was told in school that they keep the magnet from losing strength. Maybe a strong, small U shaped magnet with a keeper would work and not hurt too much other stuff nearby?