Did you broke the existing strap adjuster and you're too lazy for searching new part in shops?
You can make it on your printer! It can be printed with any fillament like PLA, ABS, PET, Nylon etc, it depends on how strong must be final part.
Why it's in 2 different objects? For easier replacement without sewing.
Model is matched to 25mm strap, designed in FreeCAD.
My facebook profile: https://www.facebook.com/fishboneworkshop/
Happy printing :D