My friend fully press the start-stop on/off button then insert a cardboard in the button gap.

Make the button forever stuck at pressed position.

It is a Mazda not sure if same applies to all car. I guess worth to try since it is the cheapest.

I don't have numerical data on the saving, but can give you some perspective on how frequent the engine runs during start-stop.

On ICE car with start-stop, not much saving on our traffic and weather conditions. Engine will start itself after mere couple of minutes due to hot weather, sometime even engine off and starts again in 10s. Only in a chill cool night, engine can stays stop for over 4-5min.

Hybrid is also doing poorly on Friday evening off work rushing home, especially in big city where sometime 20min not moving an inch, traffic light less than 100m away take 40min to 1 hour to cross.
Over 50% of the time during the idle/slow crawling engine is running also, well better than ICE with start-stop yet still poor experience. Nevertheless with engine 50% off during idling would yield some saving and start smoother than ICE start-stop due to larger motor.

So what's the point of these system? At smooth urban traffic that one can drive 40-60km/h constantly, and don't wait over 2-3min on traffic light. Then they are fantastic. I still remember one time at US, we were going to a place where Google map shown all red. But during the trip, I was like this is minor jam, then my Bangkok colleague was like this is NO JAM. I think that tells how the start-stop and hybrid shines on certain part of the world.

Lastly move up to PHEV if you hold some charge for traffic jam use, then can see the start-stop usefulness. Usually 20-40min of idling consume 1km of range, depending on the weather/exterior temperature. So hold 5km range for spare normally can last few hours of jam. Planning the trip has becoming a habit for me.