Thank you for your advives. So today I manage to remove the teak trim without tearing it appart. Behind it I found... a big nothing!
There is just the wood and a 3 to 5mm gap to the deck. The wooden box is two frames (inner and outer) glued and/or screwed so I guess it is the outer part that is attached to the deck. How exactly I`m not sure as there is no visible screw neither on top nor on the bottom.