I assume that when you tighten the knob it expands the bar that fits into the miter slot. If it isnít holding then maybe it is starting out as too loose of a fit and adding something to the width of the bar might help, bonus for something that is sticky.
If there is no room to add something to the miter bar, then maybe roughing it up with some course sandpaper might get it to hold tighter.
If all else fails, you can let your tablesaw rust, that should hold it. Just kidding.
Let us know if you come up with a solution, itís the first time Iíve heard of a hedgehog featherboard so any review, good or bad, is helpful.
In woodworking there is always more then one way to accomplish something.