A magnetic eraser that has a removable felt pad you can replace or remove to clean.
The felt pad has dimentions of roughly 300x50 mm with two 15 mm holes at the ends. It is best to mark them using already printed pieces.
The magets you'll need are 6 of 5x5x5 mm neodymium cubes (that is just what I had lying around).
To put on the felt just line up one of the holes in the felt with the hole in this middle piece called "core", then wrap it around so that the other hole lines up with the other ones. While holding it down with a finger put over it a printed "cover" and use a screw to hold it all togerher.