how to block light from computer fan in home built starter box?

I have a small bookshelf that i want to turn into a small starter/vegging room. Its kind of small so heat could be an issue even though i plan on using cfls. i still would like to put in two fans to bring in cool air and exhaust the warm air. So my question is how do i block the light that will penetrate into the box from the fans?
THanks!

Go get some small air duct piping from somewhere like lowes, home depot, or pretty much any hardware store. Fit those over the fan holes then cut a little bit bigger than a half circle of cardboard that will fit inside the duct. Make two of those and glue them to the inside apart and overlapping each other. This will block out the light but still allow all the air through.