I have been using metal boxes and magnets for my nids. I found some metal boxes from
specialist vendors online and smaller ones from the local Ace hardware. I am switching back to foam because I've had too many glue failures where the magnets come off the bases. Just had one pop off a carnifex yesterday. Not a huge issue with the plastics like warriors or gaunts, but you don't need a tyrant, hive guard or zoanthrope bouncing around in there with other models on your way to the store.