I have other words to call this but I needed a couple of draught stoppers and found this pattern in someone's blog for a door snake. It's so easy to make. Just measure the width of the door and cut a piece of fabric that width + 4" for tucking in ends x 17" long. Fold in half, right sides facing and sew together. Then put that seam in the middle and sew across one end. Turn right side out. Sew two lines an inch apart in the middle of the rectangle (this piece goes under the door) then fill with foam, stuffing, kitty litter?? or as the blog suggested a cardboard roll with batting wrapped around it. You could also fill it with polysterene beads I guess. It won't matter because the snake moves with the door so it's not going anywhere.
I've filled it with kitty litter at the moment.. cause it was cheap... and it'll do the job for now. Because they've taken all the carpet up in this house and varnished the floors the gap is about an inch