Final result wasn't pretty but in my opinion good enough. After all it's just a gluing flange and no one will ever see it. Top held in place with some woodblocks+screws to let the glue set.
Almost ready.... I have to tidy it up a bit and sand corners round etc. After that it will be covered with one piece of cloth.
To solve the problem with the opening in the coaming-storage-floor at the beam mount, I made a small 5 cm high wall in that area to prevent water in the storage compartiment to run into the main hull. Like this the water won't run into the boat, but air can flow from within the hull through the gap at te mount to the (still to cut) hatch in the coaming.
The idea is like this the coaming-storage will act as a sort of ventilation-box. I hope it's clear what I mean.