How to make my room dark easily?

I want my room dark to use my projector. I have a huge window in my room (ok, not THAT big, but about 4x5'), and I want a cheap (better yet, free) way to cover the windows. I've already tried aluminum foil, but it took to much time and effort, only to find that it left me with irritating gaps and rips which light through. Also, I don't want to pain the windows black. It sounds like a good idea, but I'm my house is a rental. The landlord wouldn't really like that. ;) Any help is appreciated! Thanks!