Just picked up this
Koomus Magnetic CD-Slot Mount for $20 from Amazon.
I installed one of the two provided metal tabs behind my Pixel XL inside its Spigen Rugged Armor (I didn't bother with the adhesive), and I can't feel any difference with the tab in place - it doesn't make the phone wobble on it's back or bow out any of the edges. While I won't use it, one could easily use the second tab with another phone, tablet, etc.
The magnet is plenty strong enough to securely hold the phone in place in portrait or landscape even driving over a cobblestone street, but not so strong that it makes it at all difficult to remove the phone from the mount. The mount itself is very compact, takes all of about 2 seconds to install/uninstall, and the bearing can be pivoted to orient the phone where it (so far) is holding it's orientation.
Right now this is getting a 10/10 rating from me, and I'm hoping that my worry about the ball loosening over time (there doesn't appear to be a way to tighten/loosen it should that happen) will prove unwarranted. Best mount I've used, to date.