Use a pressure mat with a load‑cell sensor, wire it to a small microcontroller, and code a debounce routine that only fires when the weight crosses a set threshold. Add a second sensor in parallel as a check; if either reads high you lock the door. Keep the wiring neat, use shielded cable, and test with a few different weights to calibrate. Put a mechanical override lever in the lock so you can open it if the electronics fail. That’s pretty foolproof and doesn’t need fancy tricks.