I understand your hesitation to use screen/mesh for the vents. But you may want to keep a few things in mind....what is your short term goal and what is your long term goal? I'm sure your short term is to have something ready to wear for Halloween if I remember an earlier post you made, but as you said you are hesitant due to not wanting light to bleed through and make the interior visible. For this I don't think it would be a big concern since your visor will do it's job and with as small as the vents are they won't allow too much through.
But for long term if you wish to install fans you might create more work in the long run if you put something solid there and try to remove it later. On a build of mine, I tried a particular idea and it turned out great, but as great as an idea as it was it cost me in the long run because I had to cut out and add a few hours of work that I didn't want to spare. And even if you say to yourself, "Oh I'll be careful and cut it out gently" there is always the unexpected and something may happen.
But you could also use multiple layers of screen/mesh to keep the light bleed through minimal.
But what ever choice you do make, keep your long term goal in mind. I'd hate to see you do something permanent now only to have to tear apart your helmet later. Or worse yet damage it by accident. Cause your helmet is looking very sweet as it is, and will look even better when you get your paint scheme going on.
