I purchased a cheap Bell cyclecomputer from amazon total cost was under $7 . I made a simple mount with a flat l shaped plate from lowes.
Not sure what it is called its flat shaped like an leg with 2 holes on each leg. If I remember right when I bought it for a different project it was less than $2.
Take the plat and put it in a vise with 1 leg facing down and the other just above the vise jaws . Now take a hammer and bent the top part flat to the top of the vise jaw. Take some double stick or mounting tape and mount it to the left steering arm so that the one leg gets about 1/4" from the wheel spokes. Then you use some mounting tape to hold the sensor to the spoke and mount the magnet in the same way. I sprayed some black paint I had on the plate after bending to make it look a little nicer.