This is actually a great question that turned into a better reminder!
As for a tank bag, I bought one of those Coleman/Mad Dog brand magnetic bag at Walmart for $30. It's pretty durable, has one big area with fat zippers, a button on/off clear plastic map base (that I keep my phone in - but it can get hot in there during the summer!) and a waterproof cover. It secures tight on the tank with those magnets.
As for keeping emergency contacts, I never really concerned myself with that. But now that y'all brought it up I'm going to keep some pertinent information and phone numbers on my person, probably somewhere in my jacket.
Thanks for the idea!