What's the best material to use for rear screens?
Internet Filmmakers' FAQ
There are several materials that you can use to create an effective rear screen.
If your budget is a little stretched, try using voile or scrim material. White scrim works quite well because it's very diaphanous, however it's a good idea to keep stray light off it.
You can also try getting some Rosco diffusion gel from a theatrical supply house (it's normally used for stage and film lighting). There are different grades of diffusion gel and you'll have to experiment to see which works best.
We've had some success with Rosco E-Colour+ White Diffusion #216. E-Colour+ is the European product line, but you should be able to match this up to Rosco product lines in other countries. You can also try Rosco's Cinegel #3026, which is actually the same product only slightly more expensive (due to a higher resistance to heat).
For more info on Rosco products, visit the Rosco web site.