I cut the fiberglass screen to fit over the top and both sides of the window.  If you fold the corners like you are making a bed they can be held with a safety pin and pinned against the van when you close the door.  Once inside cut off the extra.  You only need magnetic tape for the bottom of the screen.  They hold in any weather.  

PS I also have tiny window awnings the PO left in the van.  These little wonders fit snugly inside the window frame when the side screened windows are open.  They are made of cloth sewn around a coat hanger.  The hanger has to "L" shaped ends  and the whole frame is one piece of wire. The top is slightly wider than the opening of the window and the cloth is sewn tightly around the wire.  If anyone is interested I can attach a picture.
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                                     L _________  L         

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