Support bracket modified to take a sensor:

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It's not a pretty effort. In retrospect I didn't need to put the hole right at the edge, worrying about the sensor being able to pick up a target - I could have placed it further in-land (dodging the existing horizontal screw holes) and made the targets tall thin bars secured into the extrusion with countersunk machine screws to keep it low-profile. That would have been the more elegant solution, for sure. Ah well, it works and I can always make a new one later on, though I probably won't..!