For the screen, go to your local rental house and get a roll of "full diffusion" lighting gel. If I remember it's number 216 (or it may be 250, one or the other). Get a "double width" roll which will be about 8ft wide and thirty foot long, it will cost about $100 but is ideal for this purpose.

If you try to do it with material, I don't think you'll have much success in getting a sharp, defined outline.

Edit: Just checked, full white diffusion is number 216, half white is number 250. Go for the full white.