What kind of mirrors are you using? Is that tape or some kind of plates?
I use glass mirrors which are typically sold at craft stores or through Internet (e-bay). These mirrors have reflective surface covered with protective grey paint. Other mirrors, like plastic or tape, are not really flat, and their reflection is not precise. Glass mirrors can be round or square - from so-called "disco" balls (these were adopted by our forum member, Jorgek). I use round mirrors, 1/2" dia, but other slightly bigger sizes can be used, of course.
Cylindrical magnet can be glued (with epoxy or contact cement) in the center of the shiny side of the glass. This lessens the force of holding the mirror to steel dish, but is more than enough to keep the mirror in place, and makes easy for the user to remove or shift the mirror on the dish's "face".
It also assures that the reflective surface stays exactly on the dish, without any "middleman" (for example, two-sided glue tape or a magnet).
When dealing with aluminum or plastic dishes, another magnet is needed on the back side of dish.
When magnet is on the mirror, the reflection has "o" - ring, or, doughnut, shape, but it is OK.
Some designs of telescopes also accept small loss of center field, when secondary mirror is there...
There is this inconvenience, that for checking dish's correctness of its geometry, we have to depend on Sun's presence.This would be unacceptable for professionals, but for amateurs, as us, is OK...
Cheers, polgyver