HC-SR04 Ultrasonic sensor below a grid plate
This will not work if you stand on it.  Ultrasonic sensors have a minimum distance from which they can accept reflections, because the transducers themselves ring, so the circuit has to blank any reflections received while ringing is expected.  Even with distinct transmitters and receivers, there's enough electrical and/or acoustical coupling between them that they have to implement blanking.

If you did achieve a minimum distance, you'd still have the problem that the common inexpensive sensors are completely unprotected from moisture, making pointing them up (even indoors) dubious.  And no, you can't put a plastic film over them.  Weatherproof sensors however are available.

Likely you will have more luck solving this problem with some other technology - light sensors (either passive or emitter/reflector), force sensors, perhaps something capacitive, etc.  Or if you want to use ultrasonic, aim it sideways or down from above.