Nice speedo, looking forward to seeing it mounted.
I've run a bicycle computer as my speedo for the last two years. Worked fine with a magnet superglued to the front brake disc rotor. Never moved a bit in two years. However when I was stripping the wheel to have it powdercoated the magnet just slipped off.
Any magnet that is strong enough to close the reeds will work.
After hours searching on the old web found this.
http://www.trailtech.net/B011-8NS.html I ordered two and they came yesterday. Perfect fir for the disc bolts, front or back, just take one out and replace with this. More secure, permenant and look so much better than a blob of superglue.
Got mine in the UK but this is the same part number.
All the best
Chris