Cateye?
My Sigma came with a spacer for the magnet. The cateye did not, but a spacer of some sort is obviously neccessary maybe on the sensor. I had some double sided rubber/foam tape from a vintage car restoration that was about 1/8" thick. A couple of strips of something like that held in place by the zap straps might do the trick.
It looked like this:
Google double sided foam tape if you are interested.