here's my story:

the angle of my coho hatch covers didn't exactly matched the ridge angle of the deck. I realised there's no shortcut here, & I redo (redid) the covers.

after that I still didn't got a good seal, so after consulting with people here, I got to the cocnclusion that my rounded neoprene seals where too stiff, so I changed them to a different neoprene strip, a one that is softer & has a flat top, & I also put that neoprene strip on both lips: the deck & also the covers. It reduced the leak, but was still not perfect, so I narowed the strip on the deck so the presure improved. still didn't had a chance to test it since then.

the force an internal bungies can apply on the seals is limited. an external webbing can apply much more pressure, so if you can't achieve a good seal "internally", do it "externally".

twm2c

Erez