@Clint
Well, maybe it depends on the style and/or duration of playing, environment, who knows. The small hole on my Sylphyo is open since we dicussed this. Actually I forgot to glue the ring to the main case after re-assembling the instrument. Occasionally I lift the blue ring and it is always dry, so is the edge of the housing. Now I think, I should rather let the hole open for ventilation, in case some moisture comes from the electronics compartment underneath.
From a physical point of view, it seems logical to seal the hole. At least if you assume that moist air can condense on the metal ring. With my Sylphyo, however, there is no moisture on the ring, so I leave the hole open. But I have to say that I don't play sessions as long as you do and always under the same conditions in my home studio.
Perhaps I shouldn't have presented my method as a recommendation. It's just what works here.