Edit - 2nd March 2020

Added a platform with a larger hole.

Edit - 8th Jan 2020

Added holder for iPhone 11

To clarify which holder you need:

For XS Max and 11 Pro Max use iPhone_XS_Max_holder.stl
For XR and 11 (non pro) use iPhone_11_holder.stl
For X, XS, and 11 Pro use iPhone_X_and_XS_holder.stl

There are quite a few 3D scanning apps available for iPhone X, XS, and XS Max that make use of the FaceID cameras. Unfortunately, because they use the front camera it's quite difficult to get a good scan without being able to see the screen. There are various parts available that use a mirror assembly to make the process more convenient but I find they degrade the scan quality due to not being 100% reflective or scattering the dot projection.
These apps all make use of the gyro to know whether they are being rotated around an object and so a stationary phone holder with a turntable for the object won't work.

Hence this monstrosity was born.

It keeps the object stationary while you rotate the phone around a path. It seems to work pretty well. As an added bonus it even folds up for storage!

Apps I've tested it with:

Heges
STL Maker
Scandy

I've also added a phone holder that has an offset mount so that the rear camera can be used with various photogrammetry apps such as Trnio.

To complete the project you will need the following:

3 x 608ZZ skateboard bearings
5 x M5x25 bolts
5 x M5 nuts
A board or something to fit under the object on the platform

Please note, I don't have an iPhone X, only an XS Max and so I had to go by Apple's measurements for the X holder. If you make this one, let me know in the comments whether it fits properly.