I used to do this to get transparencies for printmaking. You may have a simpler way, but I have done it by working with an empty shutter on the lens board of a camera set up to look straight into the lit face of a lightbox. With my aperture wide open, I measured the actual opening and then the distance between that and the light box, divided and got the f-stop for the set-up. I took a meter reading off the lightbox, set my shutter speed and exposed a sheet that was behind a negative in a regular holder. Some holders are tighter than others. Luckily I had some that would readily accept two sheets. They are mounted face to face (emulsion to emulsion) and the resulting image will be right reading. I've done it with 4x5 and also with 8x10. Without a lens, your getting no image resolved on the film (yes, I know that the aperture is acting like a large pinhole and that it is forming an image of a sort, but the focal length for a 1"pinhole is way longer than the distance to your film, so you'll never notice it). It worked like a charm. YRMV, of course, and you may have a different idea for your light source. I had a large, clean lightbox and the blank glow was easy to work with.