I had a 4x3 electric screen with a similar problem - except I need to mask to both 16x9 and 2.5. I put a thin board covered with black velvet at the top of the screen (just to cover the metal screen housing), covered the weight at the bottom of the screen with black velvet (I didn't like the shiney black metal), then put black velvet behind the screen at the bottom. I could then raise the screen the appropriate amount to yield a 16x9 or 2.5 surface. In a darkened room it looked like masking coming up from the bottom when I raised the screen.
Of course, this is masking the bottom, but it was very effective.