Money clip rubber cement/hot glue?
I've glued strong magnets to various other things in an industrial context, and even fairly rigid substrates have a tendency to peal off the magnets.

Hot glue is good if it sticks well to both surfaces, and it often doesn't stick very well to things like metal.  It's easy to use too -- try it out on scrap materials and you'll be fine.

A much better option is to use a magnet with a hole in (searching on "magnet with hole" will find you many sizes), and fasten through the hole, whether by sewing or some other kind of fastener.  Alternatively sewing the magnet into a pocket can work well, but it doesn't sound like you've got much chance to modify the design.  This option does require strong magnets as you're putting material in between them.