Quite a few people have used that kind of masking on sensors. Your typically find those discussed in a specific project report. Some people also put a small tube over the lens of the sensor, which can be very effective for reducing the area of detection as well.
You’ll find a lot of these on the quick browse lists in the community-created wiki. Look down near the bottom of that page for the “project report” section and then you’ll see many different lists, such as one for holidays, one for impress your friends, one for pets, one for sensors, etc. 
