I'm pretty certain there no lights that have "weight" sensors. The sensors for triggering signal changes use large induction loops to detect when ferrous metal (a car frame) is near by.
A strong neodymium magnet attached to your shoe or one of your components should be enough to trigger the sensor for the lights, I've seen them work on scooters and small motorcycles.
No worries! Some people suggest using a stainless metal case around the magnet since they tend to oxidize a bit, but there are a few good guides around with ideas.