Park in Neutral

I think the way you're doing it is just fine, and is probably the best way.

Put the car in park and turn it off. Push the shiftlock down and move the lever to neutral. Get out, lock the car and it's ready to be moved as needed.

None of that is easy on a regular Prius because of the bizarre shifter, but thanks to a much more normal shifter on the Prius C - it works just like it does in any other automatic transmission car.

When the car is turned on - putting it in neutral just cuts power to both motor-generators. when the car is on, it's bad to leave it in neutral for long because the HV battery could get discharged below a safe level (the car isn't allowed to charge in neutral)

With the car turned off - the power is already off inside the transmission anyway. The only difference is that you're not using the park pawl to keep the car from moving.