Hello all. I have a Trek Emonda that is outfitted with the Bontrager Duotrap speed and cadence sensor. I really like the sensor being tucked away into the frame but I am having one major problem. The slip on cadence magnet that slides on to the crank arm is stretched too far.

I have no problem getting it on but the rubber is stretched so much that it snaps in a week. After the first snap my LBS gave me a replacement, same kind, and it just did the same thing after another week. The magnet isn't contacting anything that would rip it off. I've been really watching for that.
It just seems that my Ultegra cranks are too wide for the rubber sensor magnet.

I tried placing a rare earth magnet on the inside of the crank, sticking it to the pedal's spindle, but it's too powerful and sets off both the cadence and speed parts of the sensor.

Can anyone recommend a solution or another magnet that anyone uses? The 10mm size was perfect distance but the rubber just want large enough for the crank. I'm looking for something that will be secure yet not something that will same the crank arm.

Thanks in advance.