Hi! It looks like it would be enough. I cut a small hole (4.5 cm x 4.5 cm, which is less than 2 inches x 2 inches) into a paper and put it in front of my projector and it seems that the light went through the hole ok, when it was close to the lens.
One should however in that situation need a proper filter holder or figure out how to make one by himself at lower cost. Any ideas there? I saw this kind of stand, but that's too expensive for me: http://www.3dimagetek.com/3DFilterStand.htm
Thanks again for all the suggestions!