DMM Micro Wallnut
Here's what other sites are saying:
Protect yourself with the DMM Micro Wallnut Set when you cast off on super-thin crack climbs. These tiny nuts fit cracks down to knife blade width, and their color-coded anodized heads make it easy for you to select the right size the first time. When the cracks get so thin that you can no longer get your fingers in, sew 'em up with the DMM Micro Wallnut Set.