Here is what I used with my Trail Tech Vapor.
http://www.amazon.com/Trail-Tech-Mag...778208&sr=1-24
Except i think mine was 6mm. I drilled and tapped a hole inside of the braking surface of the front brake rotor. I used locktite and a nut to make sure it stayed put.
Very once and a while, the speedo stops working. When it does, I stop and move the ty-wraped pick-up closer to the magnet.
You just have to get creative find what will work. Those little rare earth magnets can be epoxied to almost anything.