I have tried a number of hatch hold down methods and gaskets in the past. On my latest Shooting Star project I wanted to keep it as simple as possible and decided to try Ross Leidy's clever internal bungee method. My only problem with this method was that the hatch cover would slide around a litle over the opening. I doubt it would ever slide enough to cause a serious problem, but it made me uncomfotable. I did have a little leak which may have been corrected with more tension on the bungees than I wanted to use.
I made the coming lips 1" wide and recessed them 1/8" below the inner surface of the deck to allow the lid to sit flush when the 1/4" thick x 3/8" wide gasket was compressed. The gasket is secured to the lid. So far pretty standard. The change that I made which seems to work very well is the addition of an inner coaming. I bought some "P" shaped tack on vinyl weatherstripping at an ACE hardware store. It was contact cemented down on the inner edge of the lip. I changed the hatch lid gasket to 1/4" x 3/4". The hatch lid gasket fits in the channel created by the outer edge of the hatch opening and the "P". Now when I put the hatch lid on the deck, the gasket on the hatch lid snugly fits against the "P" material and cannot slide. The hatch gasket and the "P" compress together and create a great seal and also lock the lid in place. This arrangement has proven to be very watertight with less bungee pressure than the original set up. I had the foreward hatch submerged numerous times in waves and after a number of rolls the inside of the boat was dry.

Here is a link to some photos of my SS and a couple of Outer Islands as well as the hatch gaskets.

http://www.shutterfly.com/my/os.jsp?i=67b0de21b3199cf8051e

Ken