Material:Magnets

You can salvage very strong magnets from old hard disk drives. Patrick also embedded a few ball-shaped neodymium magnets into his 3D printed bike light holder.

Most ferrous metals (those containing iron) as well as cobalt and nickel are magnetizable. High-quality stainless steel is not!

Corrosion

two corroding magnets and an intact one (on the left)

Cheap neodymium magnets are often prone to decaying after a few months of exposure even to "normal" inside air moisture levels so you might want to coat them in something that can protect them. ^[1]

Sourcing

We only have some salvaged hard drive or speaker magnets on site right now so you may want to buy your own:

• Supermagnete.de seems to be a good source, recommended by André