Technically it’s not that much of an issue because it’s kind of double layered. As in - there’s one at the top/roof level, then a big space, then a second one at indoor ceiling height. The space is surrounded by plaster, etc

The gap is just in the 2nd indoor/ceiling height one. I’m not 100% sure how the flies are getting in, but there must be a hole or something in that space that’s letting them in from wherever they’re spawning from in the roof