Is Araldite the best way to bond the speedo magnets to the propshadt?

Yes that worked for me .

worked for me too, roughen up both surfaces with emery cloth, so nice bare metal and glue 'em on.

still there after 7 years.

As above, but for extra security I wrapped them in gaffer tape and thin cable ties. OK over the top but they have never come adrift and they work perfectly.

[Edited on 7/2/16 by LBMEFM]

I used Aralidite on its own and it failed. I later combined araldite with a big tie wrap with the head of the tie at the other side of the prop to give a bit of balance.

That was years ago and I've had no problems.

I used epoxy metal, built up around and over each magnet. Has been bombproof.