Since we're talking about bags, I have been using the waterproof Ortlieb bags with dry zippers (boat people sells them).
Ortlieb Travel-Zip Zippered Duffle Drybag
I used this one in the front of my IK for a Yampa trip last April -- worked great (even upside down!) -- not as much capacity as the TRS, but waterproof -- different use -- Boat People had them on sale at the time.
Used both the 50 liter and 30 liter on my cat on a November Lodore trip (the 50 for my night gear,tent+; the 30 for a dry day bag -- the zipper sure is handy, especially for a day bag, quick grab).