The problem I think you'll have with crank magnets that attach with zip ties is slippage. I had a Cateye Double Wireless and used the magnet that came with it. It mounted nicely, but after riding on rough roads, the vibration would cause it to slide down to the bottom of crank, no matter how tight the ties were. I now use a rare earth magnet on the pedal with my sensor mounted on the stay. The Duotraps are nice but don't give you much wiggle room for the magnet positions.

My suggestion would be to search the different manufacturers web sites and look in their computer accessories for a cadence magnet type that might work for you.