At first I closed the gaps at the corners from inside with epoxy and glass and filler to get a continuous lip. Then I glued 2 mm thick epdm foam strips with thin double-sided tape, punched in correspondence with the screws. The screws must be tightened progressively the first time to compress the foam parallel to the cowl.
The foam compresses to form a sufficiently thin gasket without deforming the cover and remains elastic enough to seal the joint after several openings. the joints of the foam are just a bit superposed.
I never encountered heavy rain.
Not a drop of water entered in half an hour of light rain at 180Kn.
Claudio