That thing is essentially a magnet with a piece of tape on it. Even neodymium magnet doesn't cost that much from place like electronic parts or hobby shops. If you have old hard drives, you can take them apart and there are two inside. The magnetic field dissipates very quickly so even if you stuck this on your bottom bracket, this won't have much effect at ground level from the height of the bottom bracket. You'd have to get the thing down to about 1 inch above the road surface.

Most bicycles have aluminum rims which is better than steel rims because aluminum is more conductive. If you cannot reliably trip the light, then the sensitivity of the induction loop sensor needs to be calibrated. But you also need to know that the sensitivity for bicycle is very small. Even an inch to either side of the wire, you may not reliably trigger the light. The reason cars trip the sensors well is because they have, literally, tons of steel and aluminum in them.

The only sensors that I've really had problems triggering are the circle ones. The square/rectangle ones, you ride along the wire (if you can see it) and then stop so that both wheels are resting on the wire. Worst case scenario is you lay the bike flat on the ground on top of the sensor.