I just purchased some polarizing film off ebay. They are large sheets 19" x 39"... That was enough to cover the entire backbox of my x-files and probably 4 DMD screens. Alternately, I'm sure one piece of the film would probably cover 20 DMD screens (just guessing).
They are adhesive on one side and go on very easily. There is also a film on the front side, so don't forget to take that off.
It cut the reflection of the DMD on the playfield by probably 90%, and reduced reflection of backbox by probably 50%. This film does darken the backbox and DMD slightly, but I would say it almost improved the backbox on the x-files as it seems to have more contrast.
Here's the link...ebay.com link
as of the writing of this post, there is 152 of them available, and they cost only $26.88 per sheet with free shipping.
Please note... I am not affiliated with this seller in any way... just wanted to pass info on a great deal.