A hinge can take a lot of forms. I wouldn't expect someone to use a piano hinge on a kayak hatch and I agree with you that flexibility is desireable. Actually, the combinationg of a simple fixed tab that slid under one side of the hull with a catch on the other side would be sufficient, though some form of tether would be necessary. Flexible hinges (rubber, bungee, etc.) on either side of the tab should do the trick. Again, simplicity is the goal.