You could try "shadow" painting, which would probably be acceptable in this scale.
Paint the forward part of the "opening" as a black circle, and then create the "trough" by painting in a darker tone of the camouflage colour(s), perhaps with an even darker tone in the center.
A bit hard to explain, but I can provide a sketch if you think it will help. I used to do this, many years ago, when creating "NACA" ducts on small-scale car and aircraft models, and it looked fine.