The suggestion about an IR beam seems good at the outset, however I see a problem with fluctuations during daylight and night shots, especially if this were installed for a period of days. I remember reading about one of the Natl Geo photogs that was in a tree blind for a number of days in South America.

In addition to the thoughts of using a reflector as a reference point for the laser, if that were not your development, the other thing that would work is to set the laser up as a remote battery powered unit unconnected to your sensor and your logic unit. By doing it that way there would only be the effect of a switch closing or opening (depending on your circuitry) between your sensor and logic unit as the laser beam is interrupted. I have seen small levels that are laser and are battery powered. These are relatively inexpensive insofar as these devices are concerned. They are normally available through the building craft magazines among other sources, should you decide that is an option.

At any rate, good luck.