Sorry, I don't know about a custom tool.
When I changed this sensor I cut the old cable where it adapts into metal shroud and slid the 12pt. side of a combination wrench over the nut to loosen. I had to alternate the open end of the wrench during tightening, but was able to fasten the new sensor. This was difficult to access even on a car lift.
To possibly make fastening easier, one could cut a (sensor cable width) notch through the box end of the wrench. this would allow you to slide the new cable through the cut space and fasten the nut more easily.
