Magnetising the memory pieces
We chose medium-sized disc magnets to attach the cards.
Instead of simply glueing the magnets on the backsides (we wanted both sides of the memory cards to look good), we punched a round hole of 10 mm into every cardboard card, in which we placed the magnets.
When we bought the magnets we made sure they were of the same thickness as the memory cards.
Hence, we decided on disc magnets S-10-1.5-N
We bought the required punching press at a home improvement store for about 4 euros.