I assume your using the Bontrager Duotrap sensor for speed and cadence that fits in the stay. I have that on my Domane SL. The magnet holders that came with it did not fit and looked unsightly. I used 3M double stick take to attached the the larger magnet to the inside of the crank. Or if you lost the magnet, pickup a rare earth magnet at the store or on the web.
The 3M tape is a gray foam. Thin and comes with red backing. Made for outside use. Attach and trim the excess before removing the red backing. Clean the place you plan to attach. I used painter tape I had to hold it in place and make sure to worked before and attaching. It almost would work just stuck to pedal spindle without the tape.
Here are a couple of pictures. The larger sensor I think is the Garmin attached with double stick tape.
