carg such plastics can probably not be bent like metal, so how do you adjust it to the wearer?
I would use flexible plastic belts. You would not need any adjustment of the shape only for the length.
The shield could be adjusted accordingly from a 3D scan and printed.
carg Is the fit of the belt not more dependent on the underlying bone structure than the surface of the skin that would be measured with such a scan?
This concern has every manufacturer to face. And they doing fine with some standard shield shapes. A 3D-scan would help to change some details but it does not mean to change the whole shield shape.