I use similar magnets to those that MC71 linked too, think I use 12 in total (6 paired up) and it's held on perfectly well. Everyone who does this mod is scared that they won't be strong enough, but think how much a numberplate weighs... A few kg of force is more than enough to hold something less than half a kg in place securely. I did a shake test when doing the mod and you'd have to apply some serious force (i.e. a crash or considerable amount of air time) before it falls off.