Got it! I ordered a couple of small Cateye magnets which someone mentioned in another area of the TE forum. I like them better than the style of magnet that came with my Garmin because the zip type passes through the magnet case instead of over the top of the case. The magnet is sticky backed, but still didn't stay attached to the crank. I used the zip tie which came with it, but with the hollow back, and the crank being straight and not tapered, I still found it sliding back and forth on the crank. I took a couple small pieces of dark gray water pipe insulation foam, and shoved them between the zip tie and the back of the crank. Problem solved.

I did have a "duh!" moment on yesterday's ride. I looked down and saw that my cadence was not registering, then I realized I was "coasting." Oops.