Great input guys!
Johnathan, one of the guys I know tried wrapping the indirect tool with leather to help keep it under his knee. It didn't work too well. It seems the best way to keep it there is to sit on a low seat and elevate your foot so your knee makes more of a sharp angle and, therefore, you can get a better grip behind the knee.
Justin, I've tried notches on the end of the percussion tool and I tend to snap off large pieces off the blades. It almost looks like I've taken a bite out of the arrowhead. The tool will sometimes slide around without the notch but you don't need too much inward pressure... just change the angle of the strike a little so that more of the force goes into the blade.