That hollow stem is what I suspected was there. Sure seems like that gold cap is simply a decorative cap and the real retention nut is under the cap. That being the situation, there are cell phone mounts that would jam down that hollow center.
Still might be able use the gold cap if a hole were drilled into the center.
Then again, if that gold cap is an aftermarket nut, might be time to return to the OEM nut for a cell phone mount.