Yesterday I finished re-attaching the Rutgerson portlight - see images attached below.
I bought a warehouse leftover piece of clear UV protected 8mm polycarbonate sheet and cut out a much better fitting window panel into the stainless frame. Pro tip: use a small tooth jigsaw blade to cut the sheet in order not to chip the edges of the cut. I used a new metal blade and it worked perfectly.
I then used black Sikaflex to glue the panel to the stainless frame and then white Sikaflex to glue the portlight back in its mounting hole. Note: the original Rutgerson frame has two fixtures for the window panel, the outer Sikaflex seal and an inner molded rubber lip onto which the window panel seals. If you decide to replace your window sheet, then spare the rubber seal and do not mix it up with the identical looking Sikaflex mass.
The result is ... crystal clear
The window is better than before. I decided not to go for butyl tape as the gap between the stainless window rim and hull is large and to my feeling large gap filling is not what butyl tape is good for.
With that, I conclude this topic!