A little hard to see, but there are 5 little holes drill in the housing beside the top of the lamp. One hole where the probe sees through is 1/4" and the other 4, or 2 pairs, are 1/8" provide attachment of nylon ties to strain relief the probe wire.
Here you can see the photo diode looking down on the grid (between grids) on a 12x12. On the other side, stickies, nylon ties, and epoxy hold everything in place.