WebGL Generate CustomId

I am trying to figure out the best way to allow anonymous logins on a WebGL game built in Unity. My first thought was to go for Unity's built-in SystemInfo.deviceUniqueIdentifier, however, after testing this out, I found that this functionality is not supported in WebGL, the only alternative I can think of is to generate an id on my own and save it to the cookies (if one doesn't exist and they have chosen not to link an account explicitly).

What is the best practice for generating an ID like this dynamically that I can guarantee is unique across all of PlayFab, within the WebGL constraint.

Thanks for your help!
Chris