Measure the loader core 's outside diameter, and look for a piece of thick-walled plastic piping with slightly larger inner diameter. From the sounds of it, 1 inch or 25mm would be too small; 40 mm sounds more or less right. Cut a section as long as the width of 35 mm film, and use double-sided tape to attach the inner leader of your bulk roll to that. If you can, you can cut a longitudinal groove for the film to slide into. You'll have to work very carefully so that the roll does not unwind on you. If you have a core from a previous bulk roll, that would work best. Or if you do woodworking, it would be easy to turn out a core that is exactly the size you need.