I have negatives that were stored for 50--60 years in glassine envelopes, and they don't seem to have suffered from it. I mentioned silicone release paper because it is very smooth and is pretty much guaranteed not to stick to anything that might have gotten onto the film. If you aren't in a hurry (I needed to know quickly whether the film had anything of immediate interest) you could put the film in a folded strip of either glassine or release paper, and just store it flat under pressure for however many days, weeks or whatever that it takes to flatten out. Heat would accelerate the process, but isn't otherwise necessary. Agfa used to make strip-form glassine sleeves that were assembled like a Venetian blind, and I found one that I had left in a book for a few months. The negatives were just as flat as could be (this would have been 1970s-vintage Tri-x 35mm).