Hi !
I just installed a Duotrap S sensor in my new 8.5 DS and to get the things right, I had to cut the rubber of the biggest cadence magnet - 6 mm of the inside flat part - and then I got it placed with 4 layers of black electric tape, right behind the pedal's thread end. It is now 3 mm from the frame, working well, looking good and firm, better then just the rubber band. Let's see what solution Trek comes with.