Why not just use dry ice?

Put a fist-sized lump in the center bottom of the bucket, add your popcorn to within an inch or so of the top, put the lid on put slightly ajar to vent. After 20-30 minutes, feel the center bottom of the bucket for cold. If it's still cold, some ice is still there. Continue to feel it until the center isn't cold, then seal the lid. Watch the bucket for several minutes -- if it starts to bulge, release the lid on one side for a few minutes. Seal and watch again. When no bulging occurs, you're done.

Sue