Ze Masqued Cucumber
Protze et Chniaque !
C'est le Gomazio à crête mordorée, dont le cri annonce le soir...
Magnetizing the pieces isn't difficult with plastic. It's a little bit rougher with cardboard. The problem with the board is that a normal piece of sheet metal isn't strong enough to work well through a board.
If the pieces shape allows it (for example, hollow miniature bases), you could glue neodymium magnets under them. These magnets are very powerful and their field will be enough to work through the board up to the sheet metal. Need pre-testing though, the field might be too powerful and the pieces might be extremely hard to detach...