Go to wally world and get a closed cell foam sleeping pad for camping at like $7. Cut it up and use it. Closed cell is the key as open celled foams will rot.
I used polyethylene sheeting for covering the holes. It is cheap (think Ikea flexible cutting boards) and also very dead. Cover it with a layer or two of mat and you are set. Depending on your window clearance you can sheet metal screw it to the outside of your door or rivet it on the inside.
