To follow up on Greybeard's method (I have a cigar box full of rolls from my father, taken in the 40s and 50s stored the same, but at room temp) - How about sandwiching the film in acid free board as you suggest, but inside the board, also inside individual folds (both sides of the film) of glassine paper? I've been told that this is a good material for shielding fiber prints when flattening just as you describe, which works well for me (for prints).

I can test it on film, of course, but I've never really known the composition of glassine material, or it's possible long term effect on emulsions. The heat, as you state, is just enough to feel warm to the hand.