The ROX cadence magnet is in two parts, the magnet and a spacer. I assume you've removed the spacer to get more clearance. The magnet itself is about half an inch thick, which does seem thicker than my older CatEye magnet. I don't know whether it's common for bikes to lack a half inch of clearance between the crank and chainstays.
Is there something about your bike that prevents you from moving the sensor to a point with more clearance, or tilting it away from the magnet?
One problem with the CatEye single unit is that it must be positioned to take into account the speed sensor, whereas with separate sensors, you can move the sensor as far forward as necessary to get more clearance.