I was reading about someone doing this just yesterday... they got some cardboard, or plastic board, something like that, and taped the foil to that instead, making it easier to put the panel up and down in the window. They had a small bathroom window, so I don't know how effective that would be for you. If my apartment allowed aluminum foil on the windows I'd do this as well, but its in the lease saying I can't. Smart buggers....
Something else to try, that may not bust leases or burn grass is a thick satin finish sheet. The tight weave should stop light (if that's the need, I work nights, sleep days, so I need that) and the satin finish is semi-reflective, without being a direct mirror. It may not be as effective or cheap, but it is safer for all those outside your windows. You did mention the neighbors can't see the windows, so foil will probably work great for you.
here's a few websites I frequent to help me with ideas:
http://ikeahacker.blogspot.com/
http://treehugger.com/
http://planetgreen.discovery.com/
http://unclutterer.com/
there's a few more, but those have helped me bundles in understanding how to be ....greener? I hate that word, but for lack of a better description, there it is. How to use less energy and make a smaller impact upon the world around me. eh. You get the idea.
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