Stopping light leak from Rubbermaid box

Does anyone have a suggestion on how I can stop the light leak that comes from my exhaust fan at the top of my box? I plan on just getting something to throw over the intake down at the bottom, like a towel or something, but as far as the light that would come in from the top of the box, I dont really know how to stop that. I can build a carbon filter and attach it to that, but I'm just not sure if it would completely stop the light from coming in. I plan on also getting some black plastic to put up in my closet to ensure complete darkness. I might just have to use that to ensure the box is completely light proof. Any tips?

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Maybe use some black socks or panty hose to cover the vent. Air will still circulate but the light will be reduced.

Cool, ill try that and see what happens. I just want to make sure I dont make the plant become a male because of light leaks. Maybe I will cut a piece of plastic and put it over that fan, securing it with rubber bands