I have a BlendMount Valentine One (V1) radar detector mount available for a 981 Cayman or Boxster with the standard rearview mirror. Used only for a few months, and it's one of the best mounts on the market.

Part number BV1-2021 (please check to make sure your car is compatible): Aluminum Radar Detector Mount for Valentine One, Specialty 2021 Series, BV1-2021

Located in Gilroy, CA, but I can also ship anywhere in the US for $7.