OP hasn't logged in for 18 months and the post is over 2 years old, but I'm actually going down this rabbit hole right now.
I bought the Neewer one below but don't recommend it because it clamps onto the touchscreen of the phone (and if you watch the reflections you can see the screen bend around the clamp!), plus is seriously bulky.
The idea you need a large one isn't necessarily true - I have a range of ND filters and I found that if I hold a 46mm one up to my iPhone 17 in just the right spot it covers all the cameras with no vignetting. Having said that, as some (or all?) phones don't have apertures, you'll need enough ND to shoot wide open. I tested my iPhone 17 Pro a few days ago and discovered it needed more than 5 stops of ND in direct sun conditions, and from about 2-3 stops onwards became unusable with IR pollution, so mine is going to need my 4-stop ND, my 1-5 stop vND, and an IR cut filter.
I've just ordered the Tiffen MagSafe one (that only claims to work for the iPhone 16, not my 17) and plan to attempt to modify it to work with my phone case and see if I can get it to work, and if the 58mm filter size covers all the lenses without vignetting (especially if it doesn't align properly).