I like these mini PIR motion sensors for triggering Halloween props, so I made a housing.

• Fits the HC-SR312 mini PIR motion sensor modules (and clones).
• The sensor cavity is shaped to orient the sensor consistently.
• The cap screws over the sensor to hold it securely in place and to keep the lens array (the white plastic dome) in place.
• Slots in the housing accommodate zip-ties for mounting to posts or rails.
• The back of the housing is threaded for a PG7 cable gland with strain relief.
• The regular cap allows the dome to protrude from the end of the housing for maximum sensing area.
• The longer cap slightly restricts the angle of the detection cone but provides some protection to the plastic dome.
• The "snoot" is a cap with a small aperture for when you want to drastically restrict the detection cone.
• To reduce the sensor's range, remove the lens array and/or cover the opening of the cap/snoot with one or two layers of cellophane tape.

I've printed them in PETG and PLA.  A 0.2mm layer height works well with the threads.

Note: The cable gland on the back of the housing is not part of the model.  The housing is threaded for a PG7 cable gland purchased separately.  I tried to design a printable strain relief, but I was not successful.  In small quantities, commercial ones are inexpensive.

I've ordered the “Breadboard friendly” mini PIR sensor modules from Adafruit (https://www.adafruit.com/product/4871) and lookalikes from a couple vendors via Amazon.com.  Some vendors referred to them as HC-SR312, others as AM312.  They've all been identical and thus all work with this housing design.

For use as a general “occupancy” sensor, the short cap piece is the way to go.  If you want it to detect when enters or passes through a specific spot (to startle them with a Halloween animatronic), start with the “snoot” version of the cap and be prepared to spend some time experimenting to find the best mounting location and angle.