Thanks Brian. I had found this at a reasonable price at McMaster
High-Speed Steel Square-End End Mill for Aluminum
Uncoated, 2 Flute, 1/2" Mill Diameter, 3-1/4" Overall Length
$24.
I am in the process of drilling the 12mm center hole for the sensor starting at 1/16. The hard part was securing the fork bottom perpendicular due to the compression adjuster protruding. My pilot hole is not 100% but hopefully close enough.
Once that 12 mm hole is done I will position the sensor and draw an outline where the metal needs to be removed.referencing the attached photo, the sloped surface needs to be flat and slightly recessed I think as in second picture.