[PySide] Creating deployable GUI app

[Mark]

I think the problem is to do with PySide 2 -- or with how cx-freeze 
works with PySide 2?

When I use Python 3.6/PySide 1 (Qt 4.8), cx-freeze produces a .exe that 
runs perfectly.

When I use Python 3.6/PySide 2 (Qt 5.12) on a clone of the code base 
updated for PySide 2, cx-freeze produces a .exe that when run just pops 
up an OK dialog that says 'This application failed to start because no 
Qt platform plugin could be initialized'. Yet in the .exe's folder is a 
subfolder called plugins that contains the Qt plugins the app needs 
including platforms/qwindows.dll etc.

(I don't mind switching to PyInstaller, but that doesn't work either, 
although it pops up a dialow with a different error message.)