I've been using clamps like this for 3/8" Flex and 1/2" knockouts: . They work fine, you just tighten the two bolts and the flex cable is gripped and strain-reliefed.
Home Depot had these that were a little cheaper and use only one bolt: .
I must be doing something wrong here. The latter clamps, with the single bolt, do not grip the 3/8" flex cable even with the bolt tightened all the way down. What is the purpose of the clamp on these since the clamp is too big to grip the flex cable? I returned most of them to the store and bought the kind I'd been using. But I must be doing something really stupid for the single bolt type to not work.