After the 'water incident'
I decided to cover up the wheel-side hole in the swing arm as well.
Off-topic: If anyone askes themself, why these holes are in the swing arm, the answer is: probably because it's a casted part. Designing casts is quite tricky and even more so for intricate parts.
The hole itself is 60 x 80 mm with radii of 30 mm. So is designed a cover with 5 mm overlap and 5 mm thick for 3D printing. The bottom part is made with reduced thickness to create a narrow opening against the body to allow for aeration. I avoided clips or anything complicated and just fixed the cover with Sikaflex (silicone glue for outdoor conditions).
Admittedly the cover is totally flat and does not hug the curvature to 100 % but I did not want to make it more complicated than necessary - in the end it's hardly visible anyway and the gap is filled with silicone.
Let's see if that will work.