I have a 4:3 Model B screen. What I do is drop a piece of Duvetyne in front of the screen when watching widescreen films. This masks off the top. Since the screen drop is variable, I can adjust the bottom of the screen to fit a 1.78 or 1.85 or 2.35 AR.
When I want to watch a full frame film, I put the piece of Duvetyne behind the screen, drop the screen all the way down, and I then have a nice big 4:3 picture that makes full use of the phosphors. Simple, and (for me) effective.