I'd simply adress the door like you would a large window, and have curtains made from dark and heavy fabric, and hang them from a rod above the door, on the inside of the theater room. If you tack sashes to either side of the entrway, then the curtains can be tied off discretely when you're not watching. I did this at my old apartment to solve an acoustic problem and it was a good enough solution, with minimal investment. The lazy man's version would be to afix velcro above the door and then the velcro backing onto a piece of fabric, so you can simply put up the "door" when you watch, but it's not in the way when you're not watching.
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If you have the money - they aren't cheap, there's the Mirage screen that rejects light other than projector light; i.e.: you can watch in a bright room! Hard to believe!
See my site at
http://www.www.greathometheater.com/newsletters.html - then go to the bottom of the August newsletter.
I have no association with the company, btw. Just trying to help.
Best of luck!