They look like (or are very similar to) these:
Infinity Hobby
I have these from Infinity Hobby, and I like them. Hard to tell if they prevent cracks--I have not had any cracks since using them, but I also filled the inside of arms with Plast-aid. What I like about them is they don't protrude out much from the arm profile (barely 3mm), so my P3 still fits into the cases I have. Some of the arm reinforcement plates (like the UAV Bits ones) stick out further and they would require enlarging the arm openings in the foam case inserts. They also look pretty nice with the brushed aluminum look.
I also like Infinity Hobby's motor covers:
Infinity Hobby
I have no affiliation with Infinity, just a happy customer of these items.