Not sure if everyone has already found this yet or not, but thought I'd share it. I was at Lowe's searching for something to put in my 9.5' x 4.5' window for my projector as my shower curtain didn't quite cover it all. I almost tried a roll of plastic sheeting that looked "whitish" translucent when I saw this. It's a vinyl drop cloth that is basically the EXACT same material as a shower curtain, and it works wonderfully! The only drawback is a seam but in my case, I can hide it along the frame as my picture window is basically 3 windows together.
So if anyone is looking for something larger than a shower curtain, this will do it. It's 9x12 feet, and costs $6.