Here’s a trim panel fastener in 3d print file. I think this one is only fitted on Mk3.5, early ones are a different panel.
Item 18 on this diagram

Original on left. Printed on right;

Other way round;

Here it is installed;

Download here, .3mf in a zipped folder;

7079618 Interior Trim Panel Fastener – Sill Panel – 3D Print File
Securing pin for Fiesta Mk3.5 Interior sill panel trim.
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