I had a challenge mounting the cadence magnet on my recumbent. But I believe this could also be a problem for DF bikes as well. The problem is mounting the magnet on a crank arm that is hollowed out on the backside (which is where the magnet must be located).
The solution is to use a product called "Sugru". It's a silicone based putty that sticks to just about anything and can be sized and molded to whatever purpose you may have. It has an infinite number of uses. Once it cures, it retains its shape permanently.