Sounds good. As a former alarm company owner I am familiar with masking PIR's to control their field of view. I would mask these to a horizontal fan only a few degrees "thick" across the room to avoid lights or anything else that might cause noise.
I will probably be a bit before I start this...right now I am working on extracting an engine from a welder for use on a hovercraft.
( Oh ... and I still have a web company to attend to when I have spare time ! 