Dave

B and bloody Q.......or you can buy this system from schluter.....

1. First, adhere an aluminium bracket to the back of each perimeter tile with tile adhesive so that the magnet extends beyond the tile’s edge.
2. Set tiles with the magnets attached as perimeter limits, so that four magnets extend into the access opening.
3. To form the access panel cover, connect the corresponding number of tiles by attaching a tile to their backs with tile adhesive.
4. Using silicone, adhere the counterplates to the back of the panel cover in alignment with the perimeter magnets.
5. After the silicone has cured, install the cover and seal the surrounding joint with a colour co-ordinated sealing compound.