Since they recently added a prohibition for using opaque sun screens/shades, get these translucent cellophane sheets, available just about anywhere, and have a few of them laminated in 10 mil plastic at an office supply chain (office max, staples). Blocks sun and stiff enough to jam into a window. In the end, about $3 each.

http://m.kmart.com/cello-sheets-8-1-2inchx11inch-48-pkg-6-each-of/p-021V006123699000P?sid=KDx01192011x000001&gclid=CNW 07aCzkc0CFQmSaQodFJ8OsA