A section of inner tube
Some plastic bags
Something to seal the ends (I used a sewing machine)
Step 1: Material Prep
Shred the plastic bags. The best method I found was to pull a single bag into a elongated shape, roughly fold it in half, then in half again, and then cut 1/2"-1" (1-2 cm) slices off of the end of this bunch with a pair of scissors.
Step 2: Seal the End of the Tube
There's a wonderful instructable (https://www.instructables.com/id/IMPORTANT-TIPS-FOR-SAWING-BIKE-INNER-TUBE-SAWING-/) on sewing inner tubes, though I found that for such a short section, the oil wasn't really required, and you can sew it like any other material.
Step 3: Fill the Tube
Chase this down to the end of the tube with some sort of tamper. Apply firm pressure to compact the filling down at that end, and remove the tamper. If your tamper is not the full length of the tube (neither of mine were), just tug on the unfilled end of the tube to use the inertia of the tamper to compact the plastic bags.
Repeat these two steps until the tube is filled, then seal the remaining end of the tube shut.
Step 4: Use
This is a much less convenient than the draft blockers which are mounted on the door, but this door is seldom opened, and it does allow for more variation in the height of the floor around the door.