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.