Well don't I feel like a jerk. Completely forgot that they sell door sweeps that have bristles. Made from aluminum, the can be cut to fit and easy to drill through. You can go to home depot and feel the stiffyness. Make sure to set the just above the track and a bit past it. If you need a...
Yes picture would help to be able to see the situation and give advice.
Till then, it has already been said. Move the door up if you can. I always recommend putting the door 6" minimum from the coop floor, but if you can't do that then get it up as high as you can.
Alternately Put a board...