Thanks, everyone. I've been trying to stay away from a messenger bag - I have one, but I don't like the sweat stripe I get on my back sometimes from it. And I've tried using the included bungee to attach a bag - it's just not very well designed and doesn't hold any kind of bag I've tried very well - the last thing I need is my stuff strewn all over the street because it falls out.
Thanks for the link for the Po Campo bag - unfortunately, they seem sold out.