the trick I used was: i got a small rectangle cardboard box cut a hole on one end for a fan, and another on the back. it works kinda like the trap under your sink, creating a 90 degree duct for air that light can't backtrack through. i just mount the box with the fan pointing down and duct tape the seams. you could also use flexible tubing to achieve the same effect. here's a diagram of my "light trap". it's my own design so you could probably make some improvements to it. but it works really well.