I've tried every brand aquatic suction cup replacement I could get from online suppliers and none of them last a year in my water which is neutral city water at 7.0 out of the tap.
The suction cups from $1 stores last a couple of weeks. 3M light mounting suction cups fail quickly under water.
What I'm using now is the Zoo Med MagClip® Magnetic Suction Cups Price is awful, but you should only need one.
If you decide to DIY using hard drive magnets, wrap them in something like felt. Those magnets are are powerful enough, that if one snaps against the glass, it could create a crack.