I have the same transmission and regulatory requirement.
I mounted a B&M Micro Switch so the 5/R transmission shift arm would contact it when shifted into reverse. Like you, I had to do some creative fabrication of a mount rigid enough to ensure it continues functioning reliably and consistently over time. Mine is actually mounted on the chassis rail, as it's quite close to the shift arm, and solid motor mounts don't flex to create alignment issues.
Sorry, no photos of my mount, but it's been working for nearly 8 years now without any issues.
Here's the switch:
