I gorilla glued some to the plates...others jb weld...even others hot glue. Then I had the magnets on the inside of the vest on attached permanently at all while I got alignment right. Once I was for sure they were correctly placed (and not by just my eyes), I used a pencil to outline the plates. Then I took the vest off carefully and ensured the alignment did not shift, flip the vest roughly inside out and hot glued over the magnets to keep them in place. I have had a couple of magnets on the inside come off but it's easy to see where the magnet goes, plus you have the others keeping your alignment correct. I am almost at 150 troops and this method has been solid for me.
Just keep in mind making little pockets over the magnets, sewing them down other ways and even putting an iron on patch over top of the magnet will not work due to the metal tools you will be using. I tired all those slick ideas and hot glue was easier, faster and worked aces.