There are a lot of options for you to store you Metal tokens. And each of them is rather safe. You can choose any of the suggested below ones and be sure that your MTL will be stored as long as you want without much risk.
You can use your mobile wallets. If your gadget operates on Android, then Coinomi and Jaxx are available, for iOS you can take only Jaxx. The web wallets are also good, for example Chrome allows Jaxx. Speaking about desktop wallets we should mention Jaxx and MyEtherWalle. There are even hardware wallets like Ledger Nano S, Ledger Blue, Trezor. Be your guest and choose any option you like!