You can easily cut the shape of the magnet out of the rubber tape...
... stick it to the magnet ...
... and now the shelf can hold twice or three times as much weight.
The disc magnet S-20-10-N (20x10 mm) can hold approximately 1,75 kg in shear direction, then the steel plate starts sliding down.
If you place a rubber tape between magnet and steel plate, the disc magnet can hold approximately 5,2 kg in shear force; then it starts to slide down.
Here a test with a S-15-08-N
: The adhesive force perpendicular to the surface is 6,7 kg. The adhesive force in shear direction is only 1,45 kg.
The rubber tape causes a noticeably higher friction: The S-15-08-N
can now hold approx. 2 kg in a sideways direction.