Take a cheapo, neodymium magnet (12mmx3mm) – £5 for 10 on eBay
Glue an M5 nut onto it (using araldite or similar epoxy resin glue). The nut is really just there to make sure the magnet doesn’t slide about (or get bumped off).
Insert it, nut first into your Look pedal axle from the frame side. Below is my crank before and after adding the magnet (my old cadence magnet is still attached in the background.
Use it as a very neat cadence magnet, getting rid of the ugly, standard magnet strapped to the crank.
The finished article. Neat, hidden, out of the way, and secure. It’s easy to remove just with fingers, though you might need to have half decent nails! 🙂