These Visonic MP-840s are not only pet immune but have a sensitivity jumper that allows you to up the motion count before it will register as a triggering. That jumper often does the trick if you really have a problem with falsing.
Visonic has been in the forefront of making alarm system motion detectors that you can live with...meaning, all attempts to minimize falsing in real world settings such as the one you present. No guarantees but they are cheap enough to try. Wish I would have bought a couple more given the seller is overseas in the country that Visonic started in (before they were bought out by Tyco). Shipping & Customs was not a hassle, just took a bit.
Smart ZigBee PIR Motion Sensor MP-840 K9-85 SMA VISONIC 2.4 GHz ZigBee | eBay < Ebay
One other idea, I have put the sensor you have in a piece of PVC pipe for weather protection. 2" PVC w/ a "bell end" houses the sensor perfectly; and then leaving a bit of the pipe narrows the view-port that is sensed. They act much more like beam sensors in that configuration. However, I can't see how that would work/look very nice on your porch wall.
