Wanting to mount my phone or GPS in the truck where it would be out of the way and secure, I decided on piggybacking on the rear-view mirror mount.
From a plain aluminum sheet, I cut out a squarish hole for the protruding bits of the mirror mount and drilled three holes each for the mirror mount screws and for the Ram ball base. I sprayed the plate black. With the Ram base attached with countersunk screws and nylock nuts, I sandwiched the plate between the mirror mount and the ceiling using the original machine screws which were still plenty long.
The sheet metal where the mirror attaches is folded several times so it is plenty strong. The aluminum plate flexes a little. The whole thing looks clean, almost as if the Ram base was bolted directly into the headliner.
I haven’t tested it out on the road, but this location seems better than where most GPSs are mounted. It’s close to the driver’s field of view without blocking any part of the windshield, it gets less glare from sun coming in through the windows, it’s easy to reach with your hands, and nearer to your eyes to it’s easier to read.