Razza - if you realy want to seal the window, you need to seal the plywood real well. The best way to do that would be to put a temporary wood frame inside your existing frame - in other words, just tack some 1"x2" or whatever just inside the window frame - then attach your plywood to that with screws/nails and caulking. (There's no such thing as caulking that comes off easily after a week or two. It'll rip off anything it is attached to - wallboard, trim, whatever it is attached to. With the temp frame, the only damage is a few nail holes that are easy to patch.

If your plastic wants to fall down at the top, put a screw/nail thru a small piece of wood then into the wall and thru to a stud. You need one block in each corner and however more often you feel necessary, but this will suport the weight of the fabric without tearing it.

Do you want the airflow from your door? If you do, take the door off and cut one inch off the bottom. This will give you the equivelant of a 6" round vent thru the wall.