You might try one of the "Boundary Waters" bags from Cabela's, they have a backpack version that does have a rolltop, and a duffle bag version. I like the backpack with the shoulder straps for more utility.

http://www.cabelas.com/cabelas/en/templates/links/link.jsp?type=pod&id=0031809

Fyi - I had a similar Cabela's bag go down the Kennebeck river recently, through some Class IV and V rapids, floating, and the contents came out dry. One strap was torn, but, otherwise a pretty good indicator of the quality of their stuff. I should have included that little note initially.