I just got some Neodymium magnets from this eBay seller.

Small and flat they about become invisible on an aero spoke and being flat and thin they glue nice. He recommended mastik for the gluing as it's easily removable with a bit of solvent. As I happen to have mastik I tried it and it worked great. It's nice not having the zip tied magnet on the crank arm anymore. I do think the sensor works more reliably now with the more powerful magnets. I'll just not stick my credit card down there.