First, build in some sort of corner or rabbet/rebate for the outside edge of the film holder to fit inside of. You can do this for all but the side where the darkslide is. Then find some adhesive-backed soft black sheet foam or felt at a good craft store. Put this along the rabbet where the face of the film holder presses against the camera back. Apply pressure to keep the film holder against this seal with rubber bands, springs & bars, eccentrically rotating bars, or whatever else works for you.