I reviewed a Sealline Dry Bag a week or so back and referenced a bag made by the same company which I have had and used for nearly 20 years, and which is still going strong. This one is not made of the same heavy duty canvas-like material as that one, but is nevertheless very durable. It's effectively a  dry bag that is designed to fit into the tapered bulkhead of a kayak or sea kayak. On sea kayaking trips over the past couple of years it's been a bit of a pain to stuff sleeping bags or a tent into the tapered front of the boat, and I have ended up eschewing the use of a dry bag and going for more supple bin bags which allow you to make the most of the space. You'll appreciate that trusting a couple of bin bags to keep your sleeping bags dry is a little risky and stressful. This bag has the suppleness of a bin bag, a roll top closure, and also a clever little valve called a 'Purge Air'. This appears to be a small sponge which is integrated into the side of the bag, but which only allows air to travel in one direction - out. By closing the bag and then compressing it, air trapped inside is forced out. Because it sits proud of the surface of the bag it means that even when pressed up against the inside of the hull of the boat it will still allow air to escape. All in all a specialist, but very effective bit of kit well worth having if you need to keep stuff dry and maximise the storage space in your tapered  kit holds. It has a capacity of 20l, and weighs 220g.

More info on the Sealline website.