This is a Remix of the PIR Box here:
http://www.thingiverse.com/thing:14452

Modified to be a smaller footprint and to house a Parallax PIR Sensor (Rev B) [Product ID: 555-28027]
https://www.parallax.com/product/555-28027

There is a small cutout on the back for the pin header and wiring to snake through. This part was designed to be force fit so the PIR board can fit snuggly in the box without any other fasteners, glue etc. Worked well with Makerbot Replicator 2, but your results may vary.