You could use a UUID for this, but I think that it would probably be a bit overkill.
I would personally just do something like this with SecureRandom.hex (which is part of the ruby core)
before_create :populate_subscriber_id
private
def populate_subscriber_id
begin
self.subscriber_id = SecureRandom.hex
end while self.class.exists?(subscriber_id: subscriber_id)
end