I did it to a pair of 900's. I had that problem originally. The vibration noise for me, was the shaft not being exaactly straight. The problem with spinning backwards was what I had more trouble with. I made the cross-brace out of acrylic, like a few peole did on the RC thread, with like a tooth hanging in toward the prop. What worked best for me was shaping the tooth like the prop, same angle or at least close (\\) . That way, if the prop was to hit it going the right direction, it would push it down toward the motor. If it hit going the wrong direction, it would stop the prop, which kinda causes the motor to suck it downward. Then I made the distance between the prop and the tooth, such that it barely clears while running. Make sense?
This worked really well for me. But I still ended up taking them of the wave timer, because even though it would stop and change direction every time, eventually the prop would come loose on the shaft. Since I took them off the wave timer, they have been running great with no issues.
HTH
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