Why do you need to carry the phone in anything on the bike. Aren’t the latest iPhones (7 onwards?) waterproof? I always used to carry mine in a plastic bag or similar, but now it just goes straight into my jersey pocket.
I’ve ridden in some biblical rainstorms and it’s always been fine. You have the added benefit that it’s easily accessible if you do need to take a call (or a photo for your social media obligations )