Most metal containers like your popcorn "tin" are made of steel. Even so, they are decorative items for storing food, not for storing chemicals. "Tins" have formed seams around the bottom and at least one seam along the sides, so they aren't designed to be leak proof. The top is a friction fit and not designed to be securely air and vapor tight. If you have children and need a child resistant closure, this type of lid is not a good choice. If you still want to use a "tin" then check the metal with a magnet -- if aluminum, the magnet won't stick and I absolutely would not use it for NaOH or KOH storage. If steel, the magnet will stick, and the container would be acceptable to use.
That said, I personally wouldn't use one. The point of making a "dry bucket" is to make a secure, dry place for your lye for however long you want to store it. I don't think a metal "tin" fits the bill for me.
A plastic bucket is impervious to NaOH and KOH, leak proof, and very sturdy. The lid whether the original snap-on lid or a gamma lid, fits on tightly and securely enough that you can control the humidity inside the bucket fairly well. The lid can be made difficult to open by children.
Over the past several years, I have read about at least two incidences of NaOH containers that had cracked and spilled NaOH beads, unbeknownst to the soaper. In a dry bucket, this would have been inconvenient, but the spill would be fully contained and the external environment around the bucket would be safe. The uncontained lye caused a mess. Thankfully no one was injured, but that could have happened too.
