Here a set of lifter that is install under alpin race ski boots. those are 5mm.
I had print them in Pla, but pla is not a good plastic for that, you need a plastic that will be more resistant to abrasion. I thing if you print them with polycarbinate filament it would be much better. If you have any suggestion about and others plastic let me know. That parts need to be 100% filling, because you will walk on and when is been grind by walking it's better if it full inside.