VW accellerator cable clamp. Found at any buggy or VW parts shop.

I made my own out of a "cutoff" grade 8 bolt, the body only, no hex. Just drilled it out on a lathe and threaded all the way through. That way you can use "two allen screws, one from each side and two allen wrenches when you tighten it. We have raced with the same one for ten years.