Step 1: The Stuff!
1: I held up a piece of cardboard with tape to the outside and from the inside traced a crude outline of the window.
2: I cut a 2 layer piece of 1mil visqueen. I just layed it over the shape i traced and left about 3 inches around as i cut it.
3: Next i bought some magnet strip. (this got poor reviews for magnet strength, but i think those people were using small pieces. When you have a whole strip it works good.)
3: Adhere magnet strip to inside of your two layers of visqueen.
4: With your magnet sandwiched between your visqueen, duct tape all the edges to make it whole.
*I put a magnet strip vertically to the rear side if the window, and there's no metal there. Oops. Anyway it acts like a weight that makes it a little more firm.