Quote Originally Posted by Hornblower View Post
Consider one of these:

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

This one has a built-in semi-rigid base that makes it better for mounting on a rear rack.
I've been using this for 2 years and it has been through the worst downpours and all kinds of weather and nothing gets in that bag, solid as a rock and the hard base is a plus, highly recommended.