Not really an expert but I did experienced this a while back. I ended up painting it with a dark color as a base and then when it was dry, I painted several layers of glossy varnish over it. I thought it was effective at the time (can't see it that well on the photo though).
Recently, I saw a video of a Japanese modeler working on a 1/35th scale PT-109. For the dark windows, he opened up an old floppy disk, cut up the material inside and glued it in place. It really looked like a dark window. This technique would have probably worked on my Desert Rat's goggles.