I had a similar feeling when I first installed mine Pete....although have never come close to slipping off them.
Chris, do you find that you feet are resting on the tip ends of the blades, or squarely on the middle of the serrated section?
I do like your suggestion of longer spacer under the acorn nut on the blade tip. I could turn up a pair from aluminium or black nylon, which may be my simplest solution. Option 2 cold be to design and fabricate a "thingo" to fit within the hinge joint, to somehow limit how far the blades will swing out.