I have held off on the use of magnets on concern of accidently erasing/damaging what is stored in the metal detector (if they got too close).
I would think that you would be putting stress on the hole of the basket where mounted and that there would be galvanic action as a result of dissimilar metals (stainless steel/ aluminum) or (stainless steel /brass). I have avoided mounting one on the back of the basket as it would be hard on the foot as on sandy clean beaches I often use a scoop barefooted.
In an earlier posting, I noticed that you had a magnet on the top of the scoop(s) staff (Garrett style).
I would like to know if my concerns are unfounded and what mounting configuration works best.