I'd like to mention that I've been testing the Krypton app (iOS only for now) for U2F. You install Krypton on your iOS device, it creates keys that are only on the device. You then install the extension for Chrome. When U2F is requested they send the challenge to the iOS device which calculates the response and sends it back to the extension. App can be configured to require approval or always send response.

App also support SSH keys.

Works very well for me and the service is free. https://krypt.co/