I always believe that when you decide how to do something, it is a good idea to go ahead and do it, so:
I sawed off each half of the housing separately, after first cutting the electric cable to it. Then, I found that it didn't quite fit in the little box, so I had to carefully file off the corners. Now it fits tightly.
I think I will do some more filing to make it fit less tightly, then take it back apart - one screw now - clean it with my handy air blower (my mouth), and go back to thinking about the sealing problem.
Does anyone have an idea about how critical it is to keep the sensor dark when not in use? These always come with a cap for the sensor, which I won't be able to use, but this one also came with a nice black cloth case which should do the same thing.