I have carried a Seal Line Medium Dry Case (4 liter) in my stern hatch for over a month. It has worked well.
I use this clear case for my first-aid kit. I like to use either a red or clear container for my first-aid kit so it is immediately recognizable. I separated the contents of an Atwater Carey FA kit and placed each type of supply (medications, wipes, cold pack, instrument case, etc.) in its own ziplock plastic bag. I never rely on a single layer of waterproof material for something as critical as medical supplies. I get out solo on creeks where I won't see another person for hours-if at all-so I must be self-sufficient and have a high level of confidence in my gear.
The Seal Line case works well. The bottom is gusseted and expands. The opening was a bit "sharp" and required a touch up with fine sandpaper to relieve the edge. The case is a bit small for my kit, so I will be upgrading to the large model. This was a selection error on my part - the bag works well for its stated purpose.
I detracted one point for the rough edge on the plastic top.