[Zeke]

The only way to make a compound "crown" would be heat. If the hatch frame is flat and you want a hump, you can go N/S or E/W. I would cut some teak in a slight curve to hold the edges of your curvature on 2 sides. Silicone will definitely seal against the weather.

To do a nice silicone job, mask on both sides of the intended bead. Then dip your finger in soapy water to smooth the bead out. Pull the tape while the silicone is wet and you'll have a very nice job.