FWIW, I have also tried what these guys have done
http://www.mikeandsharonrhodes.com/halloween.htmThe problem is that with even 6mil, it will still hotspot. And if you have a high-lumen pj, it will hotspot even more. The only partial success I've had was to fold the visquine over, effectively making a 12mil screen. It will still hotspot but not so much as before. It will be fine for the kids trick or treating but I wouldn't recommend full blown movie watching with it. If the had something even thicker, perhaps that TPO roofers membrane, it might just do the trick as good rear projecting material
All projection images used with my moviemate



Oh, and since most diy are designed to do front projection, and not rear projection, all images are backwards. Please note.
Victor