: If you ... use ribs to hold it there, you will pay sometime
: in the future with white "stressed glass spots" popping up on a hot
: day.

: You can remove any force ... by sanding off the glass on the interior
: 3-6" center of the hatch cover.

You might still consider using ribs on the inside for added strength - its all
too easy to put a hatch cover down next to your boat and step on it. I used
ribs on my hatches on Geyrfugl - a Great Auk with an inverted V-section
deck, as the ridge seemed like a potential stress point if the hatch got
trodden on. I don't use an internal bungie closure, but the ribs are useful
for tie-ons to avoid losing the cover ... my ribs are made from ash - cut
from a fallen branch off one of our own trees - I see no point in buying
timber for the hundred and one odd little bits like this :-)

Andy