here's a cheap very lo-tech suggestion...
i've experimented with this and it actually works reasonably well, though not as well as an actual beam breaker...
it does work for my purposes, though...
cover the detection window of the cheap motion sensor ( i used black vinyl tape for my tests ) such that you leave an open vertical ' slit ' in the center of the window.
this narrows the sensor's ' field of view ' to just a few degrees, creating a sort of ' beam width ' detection area...
testing at about 30 feet indicated a detection ' window ' of only a few degrees that, of course, narrows as you get closer to the sensor...
the sensor became unreliable ( but still worked intermittently ) up to about 47 feet...