M_Boats: Montgomery knock down

Long ago I read about the practicality of leaving at least one hatch board in while sailing and adding them as conditions pipe up. In that same article was also the advice to install some sort of locking feature (barrel bolts) on at least the bottom companionway board. They are not handy to step over or remove to go below but they do provide a few precious minutes to get yourself sorted out after a knockdown. My 17' doesn't have any positive flotation and I have been thinking about adding some as high as I can get it so the boat will tend to float upright when swamped. Any ideas about materials available to do this? Fair winds, Tom B. Mont.17 "AS-IS"