Replacing light seals for under a dollar
Decades-old seal foam turns into black goo that leaks light onto the film. Kits exist, but a sheet of thin self-adhesive craft foam does the job for a fraction of the price.
Scrape the old foam out with a wooden toothpick, clean the channel with a little lighter fluid, then cut strips about 2 mm wide with a fresh blade and a metal ruler. Press them in with the toothpick and leave the back open for an hour before loading film.
Test with a cheap roll and a torch in a dark room before trusting it with anything important.