I had a problem with mice getting into the house through the large gap under the (rarely opened) door into the basement, so I wanted a stiff and durable barrier to stop them. Here's a free and simple fix I made.
A section of inner tube
Some plastic bags
Something to seal the ends (I used a sewing machine)
Step 1: Material Prep
Cut the inner tube to a length about 3/4" (1.5 cm) longer than the door. The exact size isn't critical, so you can just line it up against the door frame.
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.