I have the Topeak TourGuide. This is a great handlebar bag; much better than any I've owned before. I prefer it over the Ortlieb. It is smaller than it looks, but is a good size for the way I use it. Very secure attachment system. Here is a site detailing installation with pictures.
http://www.marcosnetto.com.br/bike/topeakbag.htm
You can buy it for about $43 (e.g., aebike.com).