This is a quick prove of concept:
Taped the "arm" of the
Wahoo Blue SC on the seat support in a way that the main sensor unit can rotate
inward (closer to the rail):
I used a neodymium magnet, it is self-attached to a steel bracket and the bracket is taped to the underside of the rail. Note that the magnet pole has to be aligned to the mark on the sensor and the gap between magnet and the sensor should be small about 5mm or so:
Here is another angle (from under the rail) of the magnet/bracket and the sensor
The problem of that approach is that the cadence number is double of the actual strokes per minute.