Each case has a sheet of foamboard, here black for appearance, with a sheet of magnetic sheet on top. The sheet is magnetized, but using rare earth magents on the miniatures might make that redundant. The idea is that, as magnetic sheet weakens over time, it is cheaper to redo the sheet than the minaitures. The foamboard corners are cut off as this particular type of case has very rounded corners. I buy the foamboard in bulk.