You may glue the bag on the keel position for forming but the bag is flexible. Loading lightly atop these bags eg clothing in dry bags, and bags will not come loose during rolling. Rolling with a weighted keel is easier. Filling bags to conditions gives trim. 2 bags are velcroed on top accommodating a stacked 2 bag position in hatch behind the seat's bulkhead.
What I liked about this compared to the Camelback or similar products was that this one is simply a water bladder. I'm not paying for a backpack style that I won't use. This allows me to store the bottle behind my seat in the cockpit. This keeps it out of the sun and cooler than on deck or on my back with other styles.
I found that 4l is plenty for all day, it's a larger capacity than most other products. This setup comes in at approximately $40-$60 cheaper than the backpack types.
A couple tips; run the drinking tube up inside the skirt tunnel to have it handy, and also secure the bag using its nylon webbing straps to the seatback or backband using a cheapo carabiner so it won't disappear when you dump out.