If you put a few feet of ducting on and bend it a little light shouldn't get through, if you want to get fancy you can make a light baffle, not sure if there's any diy ones on here, you can try doin a search. Basically, it's a box that you stick on the outside of your vent that has a piece of card in the middle of it that goes roughly 3 quarters down into the box, you position the box so that the air is forced down one side of the box (seperated by the card in the middle) hits the back and passes around the neck and comes out the other side, but blocking the light from passing through.