most top-down boxes are only just a hair wider than the lens that you intend to use them with. This way, there is no chance for undesired light making its way in. If you already have a top-down box that is a lot larger than your lens, you could always use a towel or something to block the light from creeping in around the sides of your lens. If you're saying that the light is coming in the sides of the box, you can always paint the sides with black paint, or something to prevent the light from getting in that way, as well.
