I've found that even 6x2.5mm N38 magnets are more than strong enough to stick pieces thick of cardboard to a whiteboard. Larger ones could probably hold someth

I've found that even 6x2.5mm N38 magnets are more than strong enough to stick pieces thick of cardboard to a whiteboard. Larger ones could probably hold something even if there's a good few millimetres of wood between the magnet and the metal board. Dangerously powerful things. I use them to keep tiny screws in place when I disassemble something. Properly placed they can even stop laptop fans completely.