I also have a set of those and they work well. I move them around a lot for different boats, and although they look like garden variety plastic they have held up well.
I also have the issue that for some boats (tandems on a small Subaru) there isn't enough room for the stops and the boat between the towers holding the crossbars to the rails. In those situations I use just two of the stops on one side (the passenger side, by habit). With that setup the stops prevent the boat from slipping to the right in the obvious way. Moreover, they also prevent the boat from slipping to the left because of the way the straps are captured in a notch on the bottom of the stop -- if the boat attempts to slip left with the right hand side strap position fixed, that requires some slack from the strap and the tension of the strap resists. Hard to describe but anyway if you don't have room to use all four, use two on one side.