Just to clarify I never recommend placing tape or anything like that on the
lens. I have found that a carefully cut out cardboard
gobo can work in a pinch. Sometimes you can find a neutral density filter used in photography that you can place infront of the projector lens and use this for protection. It is common for instance in video cameras for lens protection . Typically what you are trying to do here is to reduce the side spill over and so you are not looking for or going to get a sharp edge. Masking in the software works, but in some cases the projection of BLACK is still visible if the area is fairly large.
Sharyn