A strip of exposed 35mm film will do the trick. The stuff should be pretty cheap by the roll with digital cameras being the norm these days.
BE VERY CAREFUL! For film, you need to use "old style" B&W film, as it's the silver compounds in the exposed and developed film that acts as the filter. You need to use the overexposed end of the film, that were used in full light when first loading the 35mm cameras. If you use color film, or some of the newer B&W film that use color processing, those films don't use silver, but dye, and will not be safe to use.
Never, under any circumstances, use film negatives that have images on them.
Mark
Excellent advice. I didn’t know that B&W vs. color made a difference.