as a user of the magnets myself i used double sided tape to some perspex and put heatshrink over it and screwed a conduit clip to that. Just make sure you have the magnets sealed or you will get rust stains on the metal you are sticking them to.
Try liquid nails polyurethane (far better than the regular stuff).
This has held cabletie clips to our tiled roof for many years, nothing else came close to working. I've actually not yet found anything it won't stick together.
It's also ever so slightly flexible, just don't use to much as it expands with the moisture content from the air..and wear gloves because it sticks to skin best of all!