I have heard that clamp on forks can shift around, but have no personal experience with them.
I have an ancient set of GC brand forks that attach with a chain that wraps around the back of the bucket, and that is tensioned with a pair of small ratchet load binders. These forks do not move at all. In fact, the P.O. had bent the top of the bucket down by getting over zealous with the load binders. After straightening it, I added a 2"x2"x1/4" angle to stiffen the top edge.