Just a thought, Robert:
For a short time, we moved popcorn from one drive-in to another, using the "garbage bag" method you describe. Shortly after we started, I learned that the plastic is not "food grade", but recycled (makes sense). I was also somewhat uncomfortable, having people see us haul popcorn around that way.
Harlan Fairbanks, the concession company over here, has the big yellow bags they used when they sold pre-popped corn, and they are food grade. We've been using them for local storage when cleaning out our machines, and I feel a lot better about using them! Maybe something to check out.
They may also be able to tell you what the yield would be... that's a good question.
Jack