The default id for a remote repository is just the repo URL. However, a URL typically contains all kind of characters that are not safe for usage in local file paths. E.g. the colon ':' from the URL scheme will just blow up on Windows. The slashes from the URL also cause troubles for a path that is meant to be a simple file name instead of a directory spec.
Better choices for the default repo id could be the host name only or just some hex-encoded MD5-digest of the URL.