If the film is exposed but not yet developed, the key requirements are that the container be light-tight, clean, smooth inside, and sized so the sheets won’t get bent or scratched.

Good options mentioned were:

• a light-tight paper/film safe or safe-bag
• any similar-sized lightproof box padded with a clean soft material such as microfiber, lens cloth, velvet, or Pec-Pads
• homemade envelopes made from multiple opaque layers, such as heavy black plastic, after washing and drying it to remove factory dust
• wrapping the sheets in aluminum foil envelopes as a temporary solution

Whichever method you use, avoid dusty or rough materials, and make sure the film cannot slide around and scratch. If you make your own container, test it carefully for light leaks with a flashlight before trusting it. In general, a purpose-made light-tight film or paper safe is the simplest and safest choice.