I've used a double thickness of foam (originally started with one) and bent it in the middle in a V shape to put it in the corners. Originally (on first layer) butted the bottom and side. Seemed to improve, but not fix. I'm still thinking it through. Don't think it is the rubber bands. I'm thinking about putting a piece of masonite on top of the holder and then attaching the rubber bands. Equalizing the pressure and protecting the back side of the holder while exposing the other side. I'm think the back side may be where the light leak is occuring from possible distortion from the rubber bands.
BB