For a cheap solution, I think I can do something with these:
http://www.sure-electronics.net/download/MB-SM19116_Manual.pdf
http://www.ebay.com/itm/Non-contact-Water-proof-Ultrasonic-Motion-Detector-Security-Sensor-Module-/120980811322?pt=LH_DefaultDomain_0&hash=item1c2b04ae3a
They are cheap at $13, but fussier to source than the $100 unit I mentioned first.
I can mount the board so the sensor peers through a hole and seal it up with RTV silicone.