I probably owned two dozen French cuffed shirts when I was younger. So I've worn many, many different cuff links. What @ScuffedShoes said is true, rectangular posts resist turning. Think of those articulated cufflinks where the bar rotates through the two sides of the post. That post is often very thin compared to its width.
Another shape you might try is a small ovoid button or knob on the inside end of the post as opposed to a bar. This makes it so even if the cufflink spins, it won't fall out.
One pair I owned had a slightly curved post; this also keeps the cufflink from spinning, because the cuff is effectively thicker on the near-to-wrist side vs. the outer side.