I have the same thing. I've not had any slippage with the cowbell. I dont really go crazy on the thing though. The tambourine gets much more play and has never moved.Rhythm Tech makes this. I use it with no problems. I clamp it to a cymbal stand near my hi hat. I use both the small cowbell and the tambourine.
I did straighten the rod in a vice, and I also took said rod to my grinding wheel to make a notch that my cowbell clamp fits into. The bell would sometimes slide down the rod before I notched it. I never had to notch the rod for the tambourine, but it could be done.