As far as rigs for holding the phone in place:
Unlike everything else that Rokoko pushes far as their products go… the new head rig is… well, 80% good.
That’s probably 79% better than self 3D printed rigs. and it’s something like $1k better than the trash they listed above.
like @Enter_Reality mentioned…
After adding some extra latching to the phone I’m able to have actors do flips and pretty much everything the custom printed stuff was doing, except they are actually comfortable (their feedback, not mine, the one I printed was made for me specifically, so yea I’m comfortable either way 
).
Still eons away from real head-rigs, but let’s all remember that you are simply attempting to hold an iPhone in place - that’s the real problem…
Real head-rigs with real camera options will always allow much more mobility/stability/reliability over something made to hold an iPhone…