Unless the mask is in the focal plane of the lens, I don't think that the masking will be fully sharp. The question is: Will the masking be sharp enough?

You can do some quickie experimentation with black construction paper or black poster board. Note that, even with black stock, there will be a certain amount of reflected light that you'll need to manage.

I don't like applying something directly to the lens because the reflected light will be tightly coupled to the lens and this will result in lowered contrast.

Consider a two stage approach: Place one mask close to the lens and a second, larger mask in front of the first, spaced as far as you can tolerate.

Can you frame the screen area with black?

Full disclosure: I don't have a projector handy that would allow me to fuss with this and I've never tried any of these techniques.