I just grabbed a standard grommet for a 1/16-1/8 thickness and it worked fine. Not sure of the actual thickness. If not sure just grab a couple and take the one that doesn't fit back when you get back to the same hardware. Grommets are cheap.
Have you recently used a newer stinger in your receiver hitch. When I got a new one for a trailer, it was long enough that the sensors picked it up and sound the alarm.
